[COMMITED 2.21 2.20 2.19] S/390: Fix setcontext/swapcontext which are not restoring sigmask.

[Stefan Liebler]

Hi,

I've applied the "S/390: Fix setcontext/swapcontext which are not 
restoring sigmask." patch to release branch 2.19 - 2.21.

This patch uses sigprocmask(SIG_SETMASK) instead of SIG_BLOCK
in setcontext, swapcontext.

(cherry picked from commit 2e807f29595eb5b1e5d0decc6e356a3562ecc58e)