[libcxxabi] cleanup the use of LIBCXXABI_HAS_NO_THREADS macro (NFC)

Align the naming / use of the macro LIBCXXABI_HAS_NO_THREADS to follow what we
have in libcxx. NFC.

git-svn-id: https://llvm.org/svn/llvm-project/libcxxabi/trunk@282062 91177308-0d34-0410-b5e6-96231b3b80d8
9 files changed