MFC: r228919

MFC: r228919

Add locally implemented atomic intrinsics to libcompiler_rt.

The built-in atomic operations are not implemented in our version of GCC
4.2 for the ARM and MIPS architectures. Instead of emitting locked
instructions, they generate calls to functions that can be implemented
in the C runtime.

Only implement the atomic operations that are used by <stdatomic.h> for
datatype sizes that are supported by atomic(9). This means that on these
architectures, we can only use atomic operations on 32-bits and 64-bits
variables, which is typically sufficient.

This makes <stdatomic.h> work on all architectures except MIPS, since
MIPS still uses libgcc.

git-svn-id: svn://svn.freebsd.org/base/stable/9@236016 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f