bsd-user: Add stubs for new signal routines

bsd-user: Add stubs for new signal routines

Until the signal support is merged from the bsd-user fork, we need stubs
for cpu_loop_exit_sigsegv and cpu_loop_exit_sigbus to link. These call
abort after logging a message. Since singals aren't supported here
yet, this is sufficient.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20211102225248.52999-2-imp@bsdimp.com>
Tested-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>