I cannot find a libgcc function for this builtin. Therefor expanding it to a...

I cannot find a libgcc function for this builtin.  Therefor expanding it to a noop (which is how it use to be treated).  If someone who knows the x86 backend better than me could tell me how to get a lock prefix on an instruction, that would be nice to complete x86 support.

llvm-svn: 47213