posix: Consolidate Linux sigsuspend implementation

posix: Consolidate Linux sigsuspend implementation

This patch consolidates the sigsuspend Linux syscall generation on
sysdeps/unix/sysv/linux/sigsuspend.c.  It basically removes the alpha
assembly version which call the old sigsusped interface using only
the first doubleword from sigset. Current minimum supported kernel
on alpha (3.2) enforces rt_sigsuspend on the architecture
(__ARCH_WANT_SYS_RT_SIGSUSPEND option on kernel), so it is possible
to use the default implementation.

Checked on i686-linux-gnu, x86_64-linux-gnu, x86_64-linux-gnux32,
arch64-linux-gnu, arm-linux-gnueabihf, powerpc64le-linux-gnu,
sparc64-linux-gnu, and sparcv9-linux-gnu.

	* sysdeps/unix/sysv/linux/alpha/sigsuspend.S: Remove file.
	* sysdeps/unix/sysv/linux/sigsuspend.c: Simplify include list.
	* nptl/Makefile (CFLAGS-sigsuspend.c): New rule.
	* sysdeps/unix/sysv/linux/sparc/sparc64/Makefile
	(CFLAGS-sigsuspend.c): Remove rule.