Cleanup Linux sigqueue implementation

This patch simplify Linux sigqueue implementation by assuming
__NR_rt_sigqueueinfo existence due minimum kernel requirement
(it pre-dates Linux git inclusion for Linux 2.6.12).

Checked on x86_64-linux-gnu.

	* sysdeps/unix/sysv/linux/sigqueue.c (__sigqueue): Asssume
	__NR_rt_sigqueueinfo.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>
Reviewed-by: Zack Weinberg <zackw@panix.com>