[Patch] Make atomics work on FreeBSD+arm

[Rafael Espíndola]

> - 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.

With this the user would get an undefined reference instead of a
compiler error, no? Maybe what you need is a helper function that
classifies the type of library function to be used:

* no function available (a 33 bytes xor for example)
* a size specific function is available (4 bytes add for example)
* a size generic function is available (100 bytes load for example)

Cheers,
Rafael