Optimize sigrelse implementation

This patch simplifies sighold a bit by removing an extra sigprocmask
and using SIG_BLOCK (which union of the current set and the set argument).

Checked on x86_64-linux-gnu.

	* signal/sighold.c (sighold): Optimize implementation.

Signed-off-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>