Revert 51b0da73 "Recommit "[X86] Merge the FEATURE_64BIT and FEATURE_EM64T...
Revert 51b0da73 "Recommit "[X86] Merge the FEATURE_64BIT and FEATURE_EM64T bits in X86TargetParser.def.""
It gets miscompiled with GCC 5.3, causing Clang to crash with
"error: unknown target CPU 'x86-64'"
See the llvm-commits thread for reproduction steps.
This reverts commit 51b0da73.