[llvm] [X86] Support EGPR (R16-R31) for APX (PR #67702)

[Evgenii Kudriashov via llvm-commits]

https://github.com/e-kud commented:

```
$ clang test.c -S -o - -m32
...
test.c:2:7: error: register %r27 is only available in 64-bit mode
    2 |   asm("addq %r27, %r29");
      |       ^
```
But there is no error that `egpr` is not enabled

Also I've tried to use `m[no-]egpr`. Is it intended to not support it yet?

https://github.com/llvm/llvm-project/pull/67702