[openmp] [test] XFAIL many-microtask-args.c on ARM

On ARM, a C fallback version of __kmp_invoke_microtask is used,
which only handles up to a fixed number of arguments - while
many-microtask-args.c tests that the function can handle an
arbitrarily large number of arguments (the testcase produces 17
arguments).

On the CMake level, we can't add ${LIBOMP_ARCH} directly to
OPENMP_TEST_COMPILER_FEATURES in OpenMPTesting.cmake, since
that file is parsed before LIBOMP_ARCH is set. Instead
convert the feature list into a proper CMake list, and append
${LIBOMP_ARCH} into it before serializing it to an Python array.

Differential Revision: https://reviews.llvm.org/D138738