bsd-user/freebsd/os-syscall.c: Add get_errno and host_to_target_errno

bsd-user/freebsd/os-syscall.c: Add get_errno and host_to_target_errno

Add the helper functions get_errno and host_to_target_errno. get_errno
returns either the system call results, or the -errno when system call
indicates failure by returning -1. Host_to_target_errno returns errno
(since on FreeBSD they are the same on all architectures) along with a
comment about why it's the identity.

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