[X86] Add support for builtin to read performance monitoring counters.

This patch adds support for a new builtin instruction called
__builtin_ia32_rdpmc.

Builtin '__builtin_ia32_rdpmc' is defined as a 'GCC builtin'; on X86, it can
be used to read performance monitoring counters. It takes as input the index
of the performance counter to read, and returns the value of the specified
performance counter as a 64-bit number.

Calls to this new builtin will map to instruction RDPMC.
The index in input to the builtin call is moved to register %ECX. The result
of the builtin call is the value of the specified performance counter (RDPMC
would return that quantity in registers RDX:RAX).

This patch:
 - Adds builtin int_x86_rdpmc as a GCCBuiltin;
 - Adds a new x86 DAG node called 'RDPMC_DAG';
 - Teaches how to lower this new builtin;
 - Adds an ISel pattern to select instruction RDPMC;
 - Fixes the definition of instruction RDPMC adding %RAX and %RDX as
   implicit definitions, and adding %ECX as implicit use;
 - Adds a LLVM test to verify that the new builtin is correctly selected.