Partially revert r271012.

Incredibly weird: GCC 4.7/4.9 do support the _Noreturn and _Thread_local
keywords, but not during bootstrapping. GCC is by far the weirdest
compiler that I've ever used.

Reported by:	andreast@


git-svn-id: svn+ssh://svn.freebsd.org/base/head@271154 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f