further use of _Noreturn, for non-plain-C functions

note that POSIX does not specify these functions as _Noreturn, because
POSIX is aligned with C99, not the new C11 standard. when POSIX is
eventually updated to C11, it will almost surely give these functions
the _Noreturn attribute. for now, the actual _Noreturn keyword is not
used anyway when compiling with a c99 compiler, which is what POSIX
requires; the GCC __attribute__ is used instead if it's available,
however.

in a few places, I've added infinite for loops at the end of _Noreturn
functions to silence compiler warnings. presumably
__buildin_unreachable could achieve the same thing, but it would only
work on newer GCCs and would not be portable. the loops should have
near-zero code size cost anyway.

like the previous _Noreturn commit, this one is based on patches
contributed by philomath.

src/exit/_Exit.c

@@ -4,5 +4,5 @@
 _Noreturn void _Exit(int ec)
 {
 	__syscall(SYS_exit_group, ec);
-	__syscall(SYS_exit, ec);
+	for (;;) __syscall(SYS_exit, ec);
 }

src/exit/assert.c

@@ -1,7 +1,7 @@
 #include <stdio.h>
 #include <stdlib.h>
 
-void __assert_fail(const char *expr, const char *file, int line, const char *func)
+_Noreturn void __assert_fail(const char *expr, const char *file, int line, const char *func)
 {
 	fprintf(stderr, "Assertion failed: %s (%s: %s: %d)\n", expr, file, func, line);
 	fflush(NULL);