Fix warnings

[Roland McGrath]

> Building glibc produces lots of these warnings:
> ../include/ctype.h:27: warning: no previous prototype for `__ctype_b_loc'
> ../include/ctype.h:37: warning: no previous prototype for `__ctype_toupper_loc'
> ../include/ctype.h:47: warning: no previous prototype for `__ctype_tolower_loc'

Compiling what files?  I don't see any such warnings.  Are you using
unusual options?  What compiler?

> And also this one:
> ../sysdeps/posix/spawni.c:278: warning: control reaches end of non-void function
> 
> How can we add an  __attribute__ ((noreturn)) to _exit in include/unistd.h?

It's a GCC bug.  The attributes should be preserved by using typeof in
libc_hidden_proto, but are not.  I see Ulrich has put it in a workaround,
but I think we should have just ignored it and get GCC fixed.