Fix for BMI2 detection for Intel CPUs in fat builds

[Sven Anderson]

Hi,

Due to a bug in the BMI2 detection in mpn/x86_64/fat/fat.c (missing load of
the correct CPUID register) the codepath with the BMI2 and ADX instructions
(_coreihwl and _coreibwl suffix) is never being used in fat binaries.
Fixing it gave me a large performance boost. See attached patch.

(Please reply also directly to me, I'm not subscribed to the list.)


Best regards,

Sven
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