[X86] Add support for Read Time Stamp Counter x86 builtin intrinsics.
- Adds two new X86 builtin intrinsics ('int_x86_rdtsc' and 'int_x86_rdtscp') as GCCBuiltin intrinsics;
- Teaches the backend how to lower the two new builtins;
- Introduces a common function to lower READCYCLECOUNTER dag nodes and the two new rdtsc/rdtscp intrinsics;
- Improves (and extends) the existing x86 test 'rdtsc.ll'; now test 'rdtsc.ll' correctly verifies that both READCYCLECOUNTER and the two new intrinsics work fine for both 64bit and 32bit Subtargets.