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/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.