[libc++abi] Don't re-define _LIBCPP_HAS_NO_THREADS in single-threaded mode

Libc++ already defines the macro inside its __config_site header, so
libc++abi doesn't need to do it. Doing it just leads to -Wmacro-redefined
warnings when building libc++abi.

GitOrigin-RevId: ea23cc7120af81f52c50e1008127501da5ca63f3
1 file changed