Add support for the x86 builtin __builtin_cpu_supports.
This matches the implementation of the gcc support for the same
feature, including checking the values set up by libgcc at runtime.
The structure looks like this:
unsigned int __cpu_vendor; unsigned int __cpu_type; unsigned int __cpu_subtype; unsigned int __cpu_features;
with a set of enums to match various fields that are field out after
parsing the output of the cpuid instruction.
This also adds a set of errors checking for valid input (and cpu).
compiler-rt support for this and the other builtins in this family
(builtin_cpu_init and builtin_cpu_is) are forthcoming.