[FMV][compiler-rt] Fix cpu features initialization. (#95149)

To detect features we either use HWCAPs or directly extract system
register bitfields and compare with a value. In many cases equality
comparisons give wrong results for example FEAT_SVE is not set if SVE2
is available (see the issue #93651). I am also making the access to
__aarch64_cpu_features atomic.

The corresponding PR for the ACLE specification is
https://github.com/ARM-software/acle/pull/322.