Block TIMER_SIGNAL in sigprocmask(SIG_SETMASK, ...).

Previously, we were zeroing out the reserved signals, when we actually
wanted to have TIMER_SIGNAL always be blocked, and the other signals
always be unblocked. This resulted in process termination when a
SIGEV_THREAD timer callback calls sigprocmask(SIG_SETMASK, ...) with
any signal mask value, and then subsequently fails to complete its
callback and reach the sigtimedwait in bionic before the next timer
iteration triggers.

Add a how argument to filter_reserved_signals to appropriately
block/unblock our reserved signals.

Bug: http://b/116783733
Test: bionic-unit-tests32/64
Change-Id: Ie5339682cdeb914711cd4089cd26ee395704d0df