[PowerPC] enable atomic.c in compiler_rt and do not check and forces
lock/lock_free decisions in compiled time

Summary:
Enables atomic.c in compiler_rt and forces clang to not emit a call for runtime
decision about lock/lock_free. At compiling time, if clang can't decide if
atomic operation can be lock free, it emits calls to external functions like
__atomic_is_lock_free, __c11_atomic_is_lock_free and
__atomic_always_lock_free, postponing decision to a runtime check. According
to LLVM code documentation, the mechanism exists due to differences between
x86_64 processors that can't be decided at runtime.

On PowerPC and PowerPCSPE (32 bits), we already know in advance it can't be lock
free, so we force the decision at compile time and avoid having to implement it
in an external library.

This patch was made after 32 bit users testing the PowePC32 bit ISO reported
llvm could not be compiled with in-base llvm due to __atomic_load8 not
implemented.

Submitted by: alfredo.junior_eldorado.org.br
Reviewed by: jhibbits, dim

Differential Revision: https://reviews.freebsd.org/D22549