bsd-user: upstream signal implementation Upstream the bsd-user fork signal implementation, for the most part. This series of commits represents nearly all of the infrastructure that surround signals, except the actual system call glue (that was also reworked in the fork and needs its own series). In addition, this adds the sigsegv and sigbus code to arm. Even in the fork, we don't have good x86 signal implementation, so there's little to upstream for that at the moment. bsd-user's signal implementation is similar to linux-user's. The full context can be found in the bsd-user's fork's 'blitz branch' at https://github.com/qemu-bsd-user/qemu-bsd-user/tree/blitz which shows how these are used to implement various system calls. Since this was built from linux-user's stack stuff, evolved for BSD with the passage of a few years, it no-doubt missed some bug fixes from linux-user (though nothing obvious stood out in the quick comparison I made). After the first round of reviews, many of these improvements have been incorporated. Patchew history: https://patchew.org/QEMU/20220125012947.14974-1-imp@bsdimp.com/