include/exec: Move cpu_signal_handler declaration

include/exec: Move cpu_signal_handler declaration

There is nothing target specific about this.  The implementation
is host specific, but the declaration is 100% common.

Reviewed-By: Warner Losh <imp@bsdimp.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>