bsd-user/freebsd/os-syscall.c: Move syscall processing here

bsd-user/freebsd/os-syscall.c: Move syscall processing here

While there is some commonality between *BSD syscall processing, there's
a number of differences and the system call numbers and ABIs have been
independent since the late 90s. Move FreeBSD's proessing here and delete
it.

The upstream implementation is somewhat different than the current
implementation. It will be much easier to upstream these from scratch,
justifying the final result, rather than working out the diffs and
justifying the changes. Also tweak a comment to qemu standard form.

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