Skip lastcomm and sa tests on unsupported architectures

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