[clang] [clang] Enable --print-supported-extensions for all targets (PR #66586)

[Balint Cristian via cfe-commits]

cbalint13 wrote:

@DavidSpickett ,

> * We can limit to list flags only for a desired e.g. -mcpu=XYZ, so the user can query like:
> Let me know if it would be another interesting command-line feature, I'll be happy doing it in a subsecvent PR.

Implemented the -mcpu filter option too.
```
$ ./bin/clang --target=x86_64-unknown-linux-gnu -mcpu=sandybridge --print-supported-extensions
clang version 18.0.0 (https://github.com/cbalint13/llvm-project 4a1fe09b189f100e2fb53105bbcd7b15b8adf4f8)
Target: x86_64-unknown-linux-gnu
Thread model: posix
InstalledDir: /home/cbalint/work/GITHUB/llvm-project/build/./bin
All available -march extensions for x86-64 and -mcpu sandybridge

  64bit                                    Support 64-bit instructions
  64bit-mode                               64-bit mode (x86_64)
  avx                                      Enable AVX instructions
  cmov                                     Enable conditional move instructions
  crc32                                    Enable SSE 4.2 CRC32 instruction (used when SSE4.2 is ...
  cx16                                     64-bit with cmpxchg16b (this is true for most x86-64 chips ...
  cx8                                      Support CMPXCHG8B instructions
  false-deps-popcnt                        POPCNT has a false dependency on dest register
  fast-15bytenop                           Target can quickly decode up to 15 byte NOPs
  fast-scalar-fsqrt                        Scalar SQRT is fast (disable Newton-Raphson)
  fast-shld-rotate                         SHLD can be used as a faster rotate
  fxsr                                     Support fxsave/fxrestore instructions
  idivq-to-divl                            Use 32-bit divide for positive values less than 2^32
  macrofusion                              Various instructions can be fused with conditional branches
  mmx                                      Enable MMX instructions
  no-bypass-delay-mov                      Has no bypass delay when using the 'wrong' mov type
  nopl                                     Enable NOPL instruction (generally pentium pro+)
  pclmul                                   Enable packed carry-less multiplication instructions
  popcnt                                   Support POPCNT instruction
  sahf                                     Support LAHF and SAHF instructions in 64-bit mode
  slow-3ops-lea                            LEA instruction with 3 ops or certain registers is slow
  slow-unaligned-mem-32                    Slow unaligned 32-byte memory access
  sse                                      Enable SSE instructions
  sse2                                     Enable SSE2 instructions
  sse3                                     Enable SSE3 instructions
  sse4.1                                   Enable SSE 4.1 instructions
  sse4.2                                   Enable SSE 4.2 instructions
  ssse3                                    Enable SSSE3 instructions
  vzeroupper                               Should insert vzeroupper instructions
  x87                                      Enable X87 float instructions
  xsave                                    Support xsave instructions
  xsaveopt                                 Support xsaveopt instructions
```


https://github.com/llvm/llvm-project/pull/66586