`intrin.h` contains declarations for both `xbegin` and `_xend`, but they
should already be included transitively from `rtmintrin.h` via
`immintrin.h` and/or `x86intrin.h`. Having them in both places causes
problems if both headers are included.
Furthermore, the `intrin.h` declaration of `xbegin` seems to be bugged
anyway, since it's missing its leading underscore.
Fixes#95133
