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[1];

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.

llvm-svn: 240994