Use _Noreturn if __STDC_VERSION__ indicates C11 support.

Previously, TTHREAD_NORETURN was __attribute__((noreturn)) or nothing.
There are two reasons why this is desirable:

1) Works for compilers without GNU extension support.
2) If <stdnoreturn.h> (also part of C11) is included before
tinycthread.h, noreturn is defined as _Noreturn, so
__attribute__((noreturn)) expands to __attribute__((_Noreturn)) and
breaks.  An alternative fix is switching to __attribute__((__noreturn__)),
but may as well use the standard version.