i#2812 sigaction: support disabled vsyscall32 (#2822)
i#2812 sigaction: support disabled vsyscall32 (#2822)

This ensures that SA_RESTORER is passed to sigaction on 32-bit builds. A signal
restorer is required when vsyscall32 is disabled, which removes the default
signal restorer stub included in the vdso. Therefore, the application cannot
rely on the existence of a default signal handler, and provides one by setting
the SA_RESTORER flag.

Fixes #2812

AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio,AmesianX/dynamorio