[X86] Make -march=i686 an alias of -march=pentiumpro

I think the only reason they are different is because we don't set tune_i686 for -march=i686 to match GCC. But GCC 4.9.0 seems to have changed this behavior and they do set it now. So I think they can aliases now.

Differential Revision: https://reviews.llvm.org/D39349

llvm-svn: 316712