Use _Noreturn if __STDC_VERSION__ indicates C11 support.

Previously, TTHREAD_NORETURN was __attribute__((noreturn)) or nothing.
This is desireable as it works for compilers without GNU extension
support.

Furthermore, use __attribute__((__noreturn__)) instead of
__attribute__((noreturn)) for GCC as noreturn is defined to _Noreturn
if <stdnoreturn.h> is included before TinyCThread, which causes the
attribute to expand to an invalid value.

https://github.com/tinycthread/tinycthread/pull/16