Move ____longjmp_chk prototype to include/setjmp.h

Move ____longjmp_chk prototype to include/setjmp.h and add
attribute_hidden.

	* debug/longjmp_chk.c (____longjmp_chk): Moved to ...
	* include/setjmp.h (____longjmp_chk): Here.  Add
	attribute_hidden.