Fix a long-standing bug where Clang had a different default from GCC on

Linux and other (non-Darwin) platforms and have it use -fmath-errno by
default (for better or worse).

Darwin has seen the light here and uses -fno-math-errno by default, this
patch preserves that.

If any maintainers for a non-Linux platform would also like to opt-in to
-fno-math-errno by default, I'm happy to add folks, but we're currently
getting buts and misleading comparisons with GCC due to this difference
in behavior on Linux at least.

llvm-svn: 155607