Fix visibility of libprotobuf symbols in protoc_compiler.so on Mac

[gnossen]

Fixes #24897

@yulin-liang This will require a backport (and patch to) to v1.34.x.

Our Mac OS build currently produces many global weak symbols corresponding to static variables in libprotobuf:

000000000041a008 gw    O __DATA,__data guard variable for google::protobuf::FileDescriptorTables::GetEmptyInstance()::file_descriptor_tables
0000000000419708 gw    O __DATA,__data guard variable for google::protobuf::util::converter::ProtoStreamObjectWriter::Options::Defaults()::defaults
000000000041a198 gw    O __DATA,__data guard variable for google::protobuf::internal::ShutdownData::get()::data
0000000000209940 gw    F __TEXT,__text google::protobuf::safe_strtod(google::protobuf::StringPiece, double*)

On Mac OS, clang will produce global weak symbols for function-local static variables when the function is a class method. For example:

class Bar {                                                                                                                    
public:                                                                                                                        
  int bar() {                                                                                                                  
    static int foo;                                                                                                            
    return foo;                                                                                                                
  }                                                                                                                            
};                                                                                                                             
                                                                                                                               
int main() {                                                                                                                   
  Bar bar;                                                                                                                     
  auto foo = bar.bar();                                                                                                        
  return foo;                                                                                                                  
}  

This C++ snippet produces the following symbol table:

$ objdump -t -wide foo.o | c++filt

foo.o:	file format Mach-O 64-bit x86-64

SYMBOL TABLE:
0000000000000030 gw    F __TEXT,__text Bar::bar()
0000000000000044 gw    O __DATA,__data Bar::bar()::foo
0000000000000000 __float128     F __TEXT,__text _main

When given a choice between two weak symbols with the same name in the same process, the Mac OS runtime linker will prefer the first one:

dyld: weak bind: _protoc_compiler.cpython-36m-darwin.so:0x110772158 = _message.cpython-36m-darwin.so:guard variable for google::protobuf::FileDescriptorTables::GetEmptyInstance()::file_descriptor_tables, *0x110772158 = 0x10EE03388
dyld: weak bind: _protoc_compiler.cpython-36m-darwin.so:0x110772168 = _message.cpython-36m-darwin.so:guard variable for google::protobuf::internal::ShutdownData::get()::data, *0x110772168 = 0x10EE03370
dyld: weak bind: _protoc_compiler.cpython-36m-darwin.so:0x110772968 = _message.cpython-36m-darwin.so:google::protobuf::RepeatedField<bool>::SwapElements(int, int), *0x110772968 = 0x10ECF2CB0
dyld: weak bind: _protoc_compiler.cpython-36m-darwin.so:0x110772970 = _message.cpython-36m-darwin.so:google::protobuf::RepeatedField<bool>::Swap(google::protobuf::RepeatedField<bool>*), *0x110772970 = 0x10ECF2B50
dyld: weak bind: _protoc_compiler.cpython-36m-darwin.so:0x110772978 = _message.cpython-36m-darwin.so:google::protobuf::RepeatedField<bool>::Reserve(int), *0x110772978 = 0x10ECF1710

Above, you see references to libprotobuf symbols in _protoc_compiler.so (grpcio-tools) resolving to addresses corresponding to symbols in _message.so (protobuf wheel).

When this happens, as in #24897, the initializer for static variables may come from one version of libprotobuf while, the runtime code may come from another version of it. When data layouts or other pertinent invariants change, this may result in segmentation faults, data corruption, or other "bad behavior."

The fix in this PR is to make all symbols in _protoc_compiler.so besides _PyInit__protoc_compiler (the Python interpreter's entrypoint to the grpcio-tools C extension) local:

00000000003f87a0 lw    O __DATA,__data google::protobuf::FileDescriptorTables::GetEmptyInstance()::file_descriptor_tables
00000000003f87a8 lw    O __DATA,__data guard variable for google::protobuf::FileDescriptorTables::GetEmptyInstance()::file_descriptor_tables
00000000003f8930 lw    O __DATA,__data google::protobuf::internal::ShutdownData::get()::data
00000000003f8938 lw    O __DATA,__data guard variable for google::protobuf::internal::ShutdownData::get()::data

This issue only ever affected Mac OS. Our Linux shared object libraries contain no global weak symbols and (somewhat perplexingly), the .pyd files in our Windows artifacts contain no symbols at all, according to winedump.

[lidizheng]

This is a really nice fix! TIL.

s/#4897/#24897/ in description

[gnossen]

Known failure.