Suppress -Wempty-body warnings in GCC 6.x and later.

libc++ in LLVM 11 uses an empty else clause in
include/c++/v1/__thread_support which triggers this warning.

Reviewed by: dim, emaste
Differential Revision: https://reviews.freebsd.org/D26257

(cherry picked from commit 73fd212c813d74ad593c93904c5d8edf763e6c54)