[compiler-rt] [AArch64][compiler-rt] Add a function returning the current vector length (PR #92921)

[via llvm-commits]

yozhu wrote:

This change results in `compiler-rt` build failure:

```
compiler-rt/lib/builtins/aarch64/sme-abi-vg.c:27:5: error: call to undeclared function '__init_cpu_features'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]
    27 |     __init_cpu_features();
       |     ^
compiler-rt/lib/builtins/aarch64/sme-abi-vg.c:33:60: error: use of undeclared identifier 'FEAT_SVE'
    33 |   bool HasSVE = __aarch64_cpu_features.features & (1ULL << FEAT_SVE);
```

The new C file, `sme-abi-vg.c`, doesn't have `#if !defined(DISABLE_AARCH64_FMV)`.

https://github.com/llvm/llvm-project/pull/92921