sound: Include snd_cmi only for i386 and amd64

When running FreeBSD on an arm64/aarch64 QEMU virtual machine, using the
Intel HD Audio Controller (ich6) (intel-hda), for example, and by
following the procedure in the handbook ("Setting Up the Sound Card"):

kldload snd_driver

The following error is shown:

KLD snd_driver.ko: depends on snd_cmi - not available or version mismatch

This is because the CMedia sound driver (snd_cmi) is only built for i386
and amd64.

Add the same guards to the snd_driver metadriver.

Reviewed by: christos, emaste
Approved by: emaste (mentor)
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D47399