bsd-user: Define target_arg64

bsd-user: Define target_arg64

target_arg64 is a generic way to extract 64-bits from a pair of
arguments. On 32-bit platforms, it returns them joined together as
appropriate. On 64-bit platforms, it returns the first arg because it's
already 64-bits.

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