Add a stubbed out version of setup_sigtramp. This is not yet used for
x86, but is used for other architectures. This will be connected in
future commits.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>