bsd-user: Introduce freebsd/os-misc.h to the source tree

To preserve the copyright notice and help with the 'Author' info for
subsequent changes to the file.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Karim Taha <kariem.taha2.7@gmail.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Warner Losh <imp@bsdimp.com>
Message-Id: <20230925182709.4834-5-kariem.taha2.7@gmail.com>