[libc++abi] Change LIBCXXABI_NO_TIMER to LIBCXXABI_USE_TIMER

Instead of always defining LIBCXXABI_NO_TIMER to run the tests, only
define LIBCXXABI_USE_TIMER when we want to enable the timer. This makes
the libc++abi testing configuration simpler.

As a fly-by fix, remove the unused LIBUNWIND_NO_TIMER macro from libunwind.

Differential Revision: https://reviews.llvm.org/D111667