[Patch] Make atomics work on FreeBSD+arm

[Ed Schouten]

Hi,

After some hacking and patching, I think I now have the emission of
libcalls for C11 atomics working completely. Patch (please refresh):

http://80386.nl/pub/clang-atomic-hax.txt

So it seems Clang only had support for the generic libcalls, which
take a size, but not the optimized calls for the default sizes. As we
only have optimized calls for the atomic_fetch_* operations, we do
need to add support for optimized calls to make this actually useful.

Explanation of the patch:

- Instead of explicitly whitelisting certain sizes for the
atomic_fetch_* calls, I thought it would be sane to just assume they
are always there. In case our data types grow, we only need to update
libcompiler-rt, not the actual compiler source.
- For the other atomic ops, always use the optimized versions if size
is {1,2,4,8}, but not 16. libcompiler-rt does not provide support for
16-byte types yet, so we'd better refrain from using that one for now.
- As the operands to the fetch ops are always passed on throughout the
code by reference -- not by value -- and I don't want to make the code
more ugly, do a load of the value before passing it in as an argument
(see AddDirectArgument()).
- Change the E->isCmpXChg() check to !RetTy->isVoidType(). In other
words: if a libcall has a non-void return type, let's assume that
should be the return value of the libcall.
- While there, extend the unit tests. In addition to testing
__atomic_load(), also test __atomic_store(), to verify that
AddDirectArgument() did its job. Also add the ARM backend, so we can
test the case where all the atomics eventually do libcalls.

Thoughts? Comments?

Thanks,
--
Ed Schouten <ed at 80386.nl>