Implement GCC's `__get_cpuid_max`

[gnzlbg]

Once the cpuid.rs module is merged adding this should be trivial so we might want to mark this as an easy task for beginners. I could mentor it.

The definition of __get_cpuid_max can be found here:

https://clang.llvm.org/doxygen/cpuid_8h.html#abe7c43218d2ca542413f9fc0f5063238