Fix build for Emscripten

[schoppmp]

The current Bazel build configuration assumes that <sys/syscall.h> is present on non-windows targets, which is not the case when building with emscripten. This PR fixes the issue.

[tuankiet65]

Seems like this is only fixed for Bazel but not CMake. I have a CMake project and while compiling I got the error that syscall is not declared:

In file included from third_party/glog/src/raw_logging.cc:34:
In file included from third_party/glog/src/utilities.h:84:
build/emscripten-debug/third_party/glog/glog/logging.h:1438:68: warning: unused parameter 'usecs' [-Wunused-parameter]
                    const char* message, size_t message_len, int32 usecs) {
                                                                   ^
third_party/glog/src/raw_logging.cc:139:3: error: use of undeclared identifier 'syscall'
  safe_write(STDERR_FILENO, buffer, strlen(buffer));
  ^
third_party/glog/src/raw_logging.cc:63:34: note: expanded from macro 'safe_write'
# define safe_write(fd, s, len)  syscall(SYS_write, fd, s, len)
                                 ^
1 warning and 1 error generated.

[attilaolah]

Shortly before this code was submitted, wasm32 and wasm64 were added in bazelbuild/platforms@966defd as standard WebAssembly CPU architectures: BUILD#L83-91.

Maybe this should be changed accordingly (i.e. split into two config settings, and list them both in the select below).