For i386 kext fallback to llvm-gcc, search paths for several Darwin versions.

This replaces the hack to read UNAME_RELEASE from the environment when
identifying the OS version on Darwin, and it's more flexible.  It's also
horribly ugly, but at least this consolidates the ugliness to touch less of
the code so that it will be easier to rip out later.

llvm-svn: 140187