Tevert part of the x86 subtarget logic changes: when -march=x86-64

is given, override the subtarget settings and enable 64-bit support.
This restores the earlier behavior, and fixes regressions on
Non-64-bit-capable x86-32 hosts.

This isn't necessarily the best approach, but the most obvious
alternative is to require -mcpu=x86-64 or -mattr=+64bit to be used
with -march=x86-64 when the host doesn't have 64-bit support. This
makes things little more consistent, but it's less convenient, and
it has the practical drawback of requiring lots of test changes, so
I opted for the above approach for now.

llvm-svn: 63642