[libc++] Work around gcc/Power9 bug in `include/thread`

This fixes PR39696, which breaks the libcxx build with gcc (I tested
7.5.0) on Power9.  This fix was suggested at

https://bugs.llvm.org/show_bug.cgi?id=39696#c38

but never applied.  It just reverts 0583d9ea, which reverses
components of the original fix in 3bf63cf3, which is correct.

Fixes https://llvm.org/PR39696

Reviewed By: ldionne

Differential Revision: https://reviews.llvm.org/D81438