alpha: Use Linux generic sigaction implementation

alpha: Use Linux generic sigaction implementation

Alpha rt_sigaction syscall uses a slight different kernel ABI than
generic one:

arch/alpha/kernel/signal.c

 90 SYSCALL_DEFINE5(rt_sigaction, int, sig, const struct sigaction __user *, act,
 91                 struct sigaction __user *, oact,
 92                 size_t, sigsetsize, void __user *, restorer)

Similar as sparc, the syscall expects a restorer function.  However
different than sparc, alpha defines the restorer as the 5th argument
(sparc defines as the 4th).

This patch removes the arch-specific alpha sigaction implementation,
adapt the Linux generic one to different restore placements (through
STUB macro), and make alpha use the Linux generic kernel_sigaction
definition.

Checked on alpha-linux-gnu and x86_64-linux-gnu (for sanity).

	* sysdeps/unix/sysv/linux/alpha/Makefile: Update comment about
	__syscall_rt_sigaction.
	* sysdeps/unix/sysv/linux/alpha/kernel_sigaction.h
	(kernel_sigaction): Use Linux generic defintion.
	(STUB): Define.
	(__syscall_rt_sigreturn, __syscall_sigreturn): Add prototype.
	* sysdeps/unix/sysv/linux/alpha/rt_sigaction.S
	(__syscall_rt_sigaction): Remove implementation.
	(__syscall_sigreturn, __syscall_rt_sigreturn): Define as global and
	hidden.
	* sysdeps/unix/sysv/linux/alpha/sigaction.c: Remove file.
	* sysdeps/unix/sysv/linux/alpha/sysdep.h (INLINE_SYSCALL,
	INTERNAL_SYSCALL): Remove definitions.
	* sysdeps/unix/sysv/linux/sigaction.c: Define STUB to accept both the
	action and signal set size.
	* sysdeps/unix/sysv/linux/sparc/sparc32/sigaction.c (STUB): Redefine.
	* sysdeps/unix/sysv/linux/sparc/sparc64/sigaction.c (STUB): Likewise.