[clang][aarch64] Generate preprocessor macros for -march=armv8.6a+sve.

Summary:
The following preprocessor macros are implied when `-march=armv8.6a+sve`:

```
__ARM_FEATURE_SVE 1
__ARM_FEATURE_SVE_BF16 1
__ARM_FEATURE_SVE_MATMUL_FP32 1
__ARM_FEATURE_SVE_MATMUL_INT8 1
```

Reviewers: sdesmalen, efriedma, SjoerdMeijer, rengolin

Subscribers: tschuett, kristof.beyls, danielkiss, cfe-commits

Tags: #clang

Differential Revision: https://reviews.llvm.org/D83079