[Mingw-w64-public] __cpuid and __cpuidex prototypes

[Jeremy D. <jer...@us...>]

Recently, a build of pycryptodome failed on MSYS2's CLANGARM64, due to a
compile error about __cpuidex not being declared.  I know cpuid
is an x86 thing, and wouldn't expect it to be present on arm(64).

While investigating why this started failing, I found a few things (two of
which seem to be issues with mingw-w64).

* The prototype for __cpuid in intrin.h is present on ARM64 (and probably
ARM32 as well, not tested).  This is why the compile test that
pycryptodome was doing succeeded (they were only compiling, not linking,
or it would have failed at link time) and they went ahead and tried using
__cpuidex

* pycryptodome's test was checking for __cpuid, but actually ended up
using __cpuidex.  https://github.com/Legrandin/pycryptodome/pull/757

* There does not appear to be any prototype in intrin.h for __cpuidex
like there is for __cpuid, though it is defined in intrin-impl.h.  This
"oversight" actually allowed the above pull request to sucessfully allow
pycryptodome to build for CLANGARM64 (assuming it would have also been
erroneously prototyped on ARM as __cpuid was).