Remove hidden __libc_longjmp

Remove hidden __libc_longjmp

Since __libc_longjmp is a private interface for cancellation implementation
in libpthread, there is no need to provide hidden __libc_longjmp in libc.

Tested with build-many-glibcs.py.

	* include/setjmp.h (__libc_longjmp): Remove libc_hidden_proto.
	* setjmp/longjmp.c (__libc_longjmp): Remove libc_hidden_def.
	* sysdeps/s390/longjmp.c (__libc_longjmp): Likewise.
	* sysdeps/unix/sysv/linux/sparc/sparc64/longjmp.S (__libc_longjmp):
	Likewise.