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.
If longjmp restores the stack frame to an address which is beyond
the stack frame at the time of the longjmp call it would install
an uninitialized stack frame. If compiled with _FORTIFY_SOURCE
defined, longjmp will now bail out in this situation.