Switch to ONLY_FOR_ARCHS=amd64.

This port does not build on e.g. powerpc64 due to hardwired assumptions
about x86 e.g. in the unconditionally-built src/libxsmm_cpuid_x86.c.

Previously it had been marked BROKEN on i386 since it requires 64-bit.
So, the easiest thing to do in this case is mark it amd64-only.

Approved by: portmgr (tier-2 blanket)