Update target triple list and support status in README.md.

[alexrp]

Closes #12.
Closes #24.
Closes #111.
Closes #112.
Closes #114.

While waiting for all these builds, I went over the issues on this repo and found some more candidates for closure:

• x86_64-windows-gnu cannot build #148 (environment issue)
• Couldn't compile x86_64-linux-musl. #141 (looks stale)
• linux: Unable to compile #127 (looks stale)
• Unable to build on linux Aarch64, requires libatomic, but cannot find it #85 (looks stale)
• s390x: container 'std.os.linux.arch_bits' has no member called 'ino_t' #77 (tracked in s390x-linux-gnu #116)
• Use Python for scripting and get this working on Windows #74 (done with batch script)

[alexrp]

Notes on the state of various target triples:

• aarch64_be-linux-* is blocked on LLVM crashes on aarch64_be-linux-* for functions taking unusual vector types zig#21064 which will be fixed by the LLVM 19 upgrade.
• A consistent theme with all the arm*-linux-gnueabi*/thumb*-linux-musleabi* triples is that we run into CPU features are not passed to clang when assembling zig#10411 and arm-linux-gnueabihf #102 (comment).
  • For the former, the patch to fix this on the Clang side is really quite simple; I just need to convincingly make the case that they should take the patch.
• I have a branch that's ready to PR that replaces the arm-windows-gnu target triple with thumb-windows-gnu and does some initial porting for that target.
• loongarch64-linux-* looks to be in pretty good shape by now; we'll see how things stand after the LLVM 19 upgrade.
• I'll also take another look at all the mips64*-linux-* triples once Get mips64(el)-linux working and start testing it zig#21095 is merged and the changes have been synced here. Given the state of CI in that PR, I'm optimistic.
• powerpc-linux-gnueabi* will require resolving Building C code for powerpc-linux-gnueabi(hf) fails due to duplicate lgammal symbol zig#20376.
• riscv(32,64)-linux-gnu should start working once we complete the LLVM 19 upgrade.
• For s390x-linux-musl, I believe the only thing left to do is arch bits in std.os.linux. s390x-linux-gnu will require resolving zig cc unable to build hello-world for target s390x-linux-gnu zig#21076 (similar to the powerpc-linux-gnueabi* issue).
• It appears that LLD does not support 32-bit SPARC; this is a blocker for sparc-linux-gnu. Great opportunity for ZLD!
• There are several LLVM issues blocking sparc64-linux-gnu.
  • [SPARC] Crash in SparcAsmPrinter::printOperand() when using function symbol as inline asm input llvm/llvm-project#103493 (fixed in LLVM 19)
  • [SPARC] Assembler does not recognize illtrap llvm/llvm-project#105639
  • sparc64-linux-gnu #172 (comment) (need to create a repro and create an upstream issue)
• As I noted here, the thumb*-linux-gnueabi* triples seem to not make sense. I have a branch ready that removes those.
• ILP32 ABIs like mips64*-linux-gnuabin32 and x86_64-linux-gnux32 are very, very broken still. I do plan to get around to these, but they're low on the priority list because almost nobody uses them.

(arc-linux-gnu, csky-linux-gnueabi*, and m68k-linux-* are all still experimental targets in LLVM, so not relevant here.)