Recommit "[X86] Merge the FEATURE_64BIT and FEATURE_EM64T bits in X86TargetParser.def."

These represent the same thing but 64BIT only showed up from
getHostCPUFeatures providing a list of featuers to clang. While
EM64T showed up from getting the features for a named CPU.

EM64T didn't have a string specifically so it would not be passed
up to clang when getting features for a named CPU. While 64bit
needed a name since that's how it is index.

Merge them by filtering 64bit out before sending features to clang
for named CPUs.