Driver: Turn the default value for -fmath-errno into a proper target hook and...

Driver: Turn the default value for -fmath-errno into a proper target hook and disable it by default on more platforms.

For now -fno-math-errno is the default on BSD-derived platforms (Darwin,
DragonFlyBSD, FreeBSD, NetBSD, OpenBSD). If the default is not right for
your platform, please yell. I only verified the result with the default
compilers on Darwin and FreeBSD.

llvm-svn: 155990