[X86] Add Clang support for intrinsics __rdtsc and __rdtscp.

This patch:
 1. Adds a definition for two new GCCBuiltins in BuiltinsX86.def:
   __builtin_ia32_rdtsc;
   __builtin_ia32_rdtscp;

 2. Replaces the already existing definition of intrinsic __rdtsc in
    ia32intrin.h with a simple call to the new GCC builtin __builtin_ia32_rdtsc.

 3. Adds a definition for the new intrinsic __rdtscp in ia32intrin.h