[libc++abi] Use from-scratch testing configs for libc++abi by default

Like we have been doing for libc++ for a while now, start using
from-scratch testing configurations for libc++abi.

As a fly-by fix, remove the LIBCXXABI_NO_TIMER macro, which was defined
but never used.

Differential Revision: https://reviews.llvm.org/D125242