Change Feature64Bit to not imply FeatureSSE2. All x86-64 hardware has

SSE2, however it's possible to disable SSE2, and the subtarget support
code thinks that if 64-bit implies SSE2 and SSE2 is disabled then
64-bit should also be disabled. Instead, just mark all the 64-bit
subtargets as explicitly supporting SSE2.

Also, move the code that makes -march=x86-64 enable 64-bit support by
default to only apply when there is no explicit subtarget. If you
need to specify a subtarget and you want 64-bit code, you'll need to
select a subtarget that supports 64-bit code.

llvm-svn: 63575