[OpenMP] libomp: disable definitions of 5.1 atomics for non-x86 arch.

Declarations of 5.1 atomic entries were added under
"#if KMP_ARCH_X86 || KMP_ARCH_X86_64" in kmp_atomic.h,
but definitions of the functions missed architecture guard in kmp_atomic.cpp.
As a result mangled symbols were available on non-x86 architecture.
The patch eliminates these unexpected symbols from the library.

Differential Revision: https://reviews.llvm.org/D112261