With SA_RESTART (#2659 (closed)), we cannot distinguish not-yet-executed-syscall from syscall-was-interrupted-in-the-kernel. This doesn't matter for most syscalls, but it does matter for sigreturn, whose asynch_target points somewhere other than right after the syscall, so we exclude it (it can't be interrupted so we know we haven't executed it yet).
Fixes #2995 (closed)