bsd-user: move TARGET_MC_GET_CLEAR_RET to target_os_signal.h

Move TARGET_MC_GET_CLEAR_RET to freebsd/target_os_signal.h since it's
architecture agnostic on FreeBSD.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>