[openmp] Fix build break for less common architectures

fb947c35 introduced the gas
macro COMMON, but it was only defined within ifdefs of the form:

    #if (KMP_OS_LINUX || KMP_OS_DARWIN || KMP_OS_WINDOWS) && KMP_ARCH_AARCH64

It was used, however, within other conditions:

    #if KMP_ARCH_ARM || KMP_ARCH_MIPS

and:

    #if KMP_ARCH_PPC64 || KMP_ARCH_AARCH64 || KMP_ARCH_MIPS64 || KMP_ARCH_RISCV64 || KMP_ARCH_LOONGARCH64

Move the definition of the COMMON macro out from the current ifdef,
so that it always gets defined (as it's only dependent on the target
platform).

This fixes building on ARM (and presumably all the other mentioned
architectures except aarch64).

Differential Revision: https://reviews.llvm.org/D138703