Documentation: Build mic/mpssd only for x86_64

mic/mpssd along with MIC drivers are currently only usable on
x86_64. So build mic/mpssd only for x86_64 to avoid build breaks on
big-endian systems.

Reported-by: Daniel Borkmann <dborkman@redhat.com>
Reported-by: Dan Streetman <ddstreet@gmail.com>
Suggested-by: Peter Foley <pefoley2@pefoley.com>
Signed-off-by: Ashutosh Dixit <ashutosh.dixit@intel.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>