sigprocmask handling not being invoked on Mac

While trying to debug the linux.sigaction test on Mac for PR #5255, which uses SYS_sigprocmask (instead of SYS_rt_sigprocmask), I found that the pre and post syscall handling wasn't being invoked at all. This needs further investigation. When done, the linux.sigaction test should be enabled on MacOS like PR #5255 did before 2e44926d.