Reland "sigchain: add wrappers for sigaction64/sigprocmask64."

This reverts commit bed29657.

"Fix" the mac build by using sigchain_dummy on it unconditionally.

Also, fix a test breakage caused by use of memcmp on a struct with
padding bytes that went from being assigned via memcpy to manual
assignment of each of the fields.

Test: test/testrunner/testrunner.py -b --target -t 004-SignalTest
Change-Id: I332d633a06abbf189d1e51fa8d031c2745f8f1a7