Call out to GCC-compatible runtime functions for atomic ops that we can't use

Call out to GCC-compatible runtime functions for atomic ops that we can't use
LLVM intrinsics for.

I have an implementation of these functions, which wants to go in a libgcc_s
equivalent in compiler-rt.  It's currently here:

http://people.freebsd.org/~theraven/atomic.c

It will be committed to compiler-rt as soon as I work out where would be a
sensible place to put it...

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@153666 91177308-0d34-0410-b5e6-96231b3b80d8