Improve "sysnums" test (signal_pre_syscall.c).

The test is rewritten to repeatedly adjust the timer so that it hits
the critical section more often. On AArch64 without DEBUG the new test
ran in about 0.5 s instead of about 15 s and hit the critical section
about 400 times instead of about 1 time.

Change-Id: I96b51e817b57dddaabf9ece193d333d78085f84c