[PATCH] x86: Adjust tst-cpu-features-supports.c for GCC 11

[Florian Weimer]

* H. J. Lu via Libc-alpha:

> Set RDRAND usable if CPU supports RDRAND.  Also check HAS_CPU_FEATURE
> instead of CPU_FEATURE_USABLE for FSGSBASE, LM and XSAVES since
> FSGSBASE/LM require kernel support and XSAVES is supervisor-mode only.

I think the commit subject is misleading because the RDRAND part is a
glibc bug fix.  Rest looks okay to me.

Thanks,
Florian
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