[PATCH] x86: Install <sys/platform/x86.h> [BZ #26124]

[Florian Weimer]

* H. J. Lu via Libc-alpha:

> <sys/platform/x86.h> exports only:
>
> struct cpu_features
> {
>   struct cpuid_registers cpuid[COMMON_CPUID_INDEX_MAX];
>   unsigned int feature[FEATURE_INDEX_MAX];
>   struct cpu_features_basic basic;
> };

The struct cpu_features ABI does not appear to be stable, as I wrote on
the other thread:

  <https://sourceware.org/pipermail/libc-alpha/2020-June/115161.html>

Thanks,
Florian