remove unnecessary and problematic _Noreturn from crt/ldso startup

after commit a48ccc15 removed the use
of _Noreturn on the stage3_func type (which only worked due to it
being defined to the "GNU C" attribute in C99 mode), GCC could no
longer assume that the ends of __dls2 and __dls2b are unreachable, and
produced a warning that a function marked _Noreturn returns.

also, since commit 4390383b, the
_Noreturn declaration for __libc_start_main in crt1/rcrt1 has been not
only inconsistent with the definition, but wrong. formally,
__libc_start_main does return, via a (hopefully) tail call to a helper
function after the barrier. incorrect usage of _Noreturn in the
declaration was probably formal UB.

the _Noreturn specifiers were not useful in any of these places, so
remove them all. now, the only remaining usage of _Noreturn is in
public interfaces where _Noreturn is part of their contract.

crt/crt1.c

@@ -8,7 +8,7 @@
 int main();
 weak void _init();
 weak void _fini();
-_Noreturn int __libc_start_main(int (*)(), int, char **,
+int __libc_start_main(int (*)(), int, char **,
 	void (*)(), void(*)(), void(*)());
 
 void _start_c(long *p)

crt/rcrt1.c

@@ -5,10 +5,10 @@
 int main();
 weak void _init();
 weak void _fini();
-_Noreturn int __libc_start_main(int (*)(), int, char **,
+int __libc_start_main(int (*)(), int, char **,
 	void (*)(), void(*)(), void(*)());
 
-hidden _Noreturn void __dls2(unsigned char *base, size_t *sp)
+hidden void __dls2(unsigned char *base, size_t *sp)
 {
 	__libc_start_main(main, *sp, (void *)(sp+1), _init, _fini, 0);
 }

ldso/dynlink.c

@@ -1644,7 +1644,7 @@ hidden void __dls2(unsigned char *base, size_t *sp)
  * so that loads of the thread pointer and &errno can be pure/const and
  * thereby hoistable. */
 
-_Noreturn void __dls2b(size_t *sp)
+void __dls2b(size_t *sp)
 {
 	/* Setup early thread pointer in builtin_tls for ldso/libc itself to
 	 * use during dynamic linking. If possible it will also serve as the
@@ -1665,7 +1665,7 @@ _Noreturn void __dls2b(size_t *sp)
  * process dependencies and relocations for the main application and
  * transfer control to its entry point. */
 
-_Noreturn void __dls3(size_t *sp)
+void __dls3(size_t *sp)
 {
 	static struct dso app, vdso;
 	size_t aux[AUX_CNT], *auxv;