linux-user/host/aarch64: Populate host_signal.h

Split host_signal_pc and host_signal_write out of user-exec.c.
Drop the *BSD code, to be re-created under bsd-user/ later.

Reviewed-by: Warner Losh <imp@bsdimp.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
2 files changed