[libcxxabi] Stub out 'sleep' call when _LIBCXXABI_HAS_NO_THREADS is defined.

While running this test on a bare metal target, I got an error as 'sleep' was not available on that system. As 'sleep' call is not doing anything useful for cases when _LIBCXXABI_HAS_NO_THREADS is defined. This patch puts it under this check.

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D89871