[clang] Change AMX macros to match names from GCC

The current behavior for AMX macros is:

```
gcc -march=native -dM -E - < /dev/null | grep TILE

clang -march=native -dM -E - < /dev/null | grep TILE
```

which is not ideal.  Change `__AMXTILE__` and friends to `__AMX_TILE__` (i.e.
have an underscore in them).  This makes GCC and Clang agree on the naming of
these AMX macros to simplify downstream user code.

Fix this for `__AMXTILE__`, `__AMX_INT8__`, `__AMX_BF16__`, and `__AMX_FP16__`.

Differential Revision: https://reviews.llvm.org/D143094