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 <[e-mail]>
Reported-by: Dan Streetman <[e-mail]>
Suggested-by: Peter Foley <[e-mail]>
Signed-off-by: Ashutosh Dixit <[e-mail]>
Signed-off-by: Jonathan Corbet <[e-mail]>

Documentation/mic/mpssd/Makefile

···
 # List of programs to build
-hostprogs-y := mpssd
+hostprogs-$(CONFIG_X86_64) := mpssd
 
 mpssd-objs := mpssd.o sysfs.o