Documentation/x86: Add an example program for AMX usage

Add an example program that demonstrates use of the arch_prctl based API
for dynamically requesting the use of AMX instructions, and then
performing a simple matrix dot product.

Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Chang Seok Bae <chang.seok.bae@intel.com>
Signed-off-by: Vishal Verma <vishal.l.verma@intel.com>