Skip lastcomm and sa tests on unsupported architectures

usr.bin/lastcom/tests/Makefile
usr.sbin/sa/tests/Makefile
Set allow_architectures appropriately. These tests depend on golden
files that must be generated for each architecture, and haven't yet
been generated for all of them.

PR: 210566
PR: 204154
Reviewed by: ngie
Approved by: re (gjb)
MFC after: 4 weeks
Sponsored by: Spectra Logic Corp
Differential Revision: https://reviews.freebsd.org/D6960