[X86] Add 'fxsr' to the getHostCPUFeatures detection code.

We implicitly mark this feature as enabled when the target is 64-bits, but our detection code for -march=native didn't support it so you can't detect it on 32-bit targets.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@353963 91177308-0d34-0410-b5e6-96231b3b80d8