Hide internal signal functions [BZ #18822]

Hide internal signal functions [BZ #18822]

Hide internal signal functions to allow direct access within libc.so and
libc.a without using GOT nor PLT.

__GI___kill and __GI___sigaltstack are defined when syscalls.list is
used to generate them.  Otherwise libc_hidden_def is needed explicitly.

	[BZ #18822]
	* include/signal.h (__kill): Add libc_hidden_proto.
	(__sigblock): Likewise.
	(__sigprocmask): Likewise.
	(__sigaltstack): Likewise.
	* signal/kill.c (__kill): Add libc_hidden_def.
	* signal/sigblock.c (__sigblock): Likewise.
	* signal/sigprocmask.c (__sigprocmask): Likewise.
	* sysdeps/mach/hurd/kill.c (__kill): Likewise.
	* sysdeps/mach/hurd/sigaltstack.c (__sigaltstack): Likewise.
	* sysdeps/mach/hurd/sigprocmask.c (__sigprocmask): Likewise.
	* sysdeps/posix/sigblock.c (__sigblock): Likewise.
	* sysdeps/unix/sysv/linux/alpha/sigprocmask.c (__sigprocmask):
	Likewise.
	* sysdeps/unix/sysv/linux/ia64/sigprocmask.c (__sigprocmask):
	Likewise.
	* sysdeps/unix/sysv/linux/s390/s390-64/sigprocmask.c
	(__sigprocmask): Likewise.
	* sysdeps/unix/sysv/linux/sigprocmask.c (__sigprocmask):
	Likewise.
	* /sysdeps/unix/sysv/linux/sparc/sparc64/sigprocmask.c
	(__sigprocmask): Likewise.
	* sysdeps/unix/sysv/linux/x86_64/sigprocmask.c (__sigprocmask):
	Likewise.