bsd-user: Move stack initializtion into a per-os file.

Move all of the stack initialization into target_os_stack.h. Each BSD
sets up processes a little differently.

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