SYSTEM_COMPILER: Rework the logic to allow a 'make test-system-compiler'.

1. Always calculate what the expected values are.
2. Add 'make test-system-compiler' to show all of the computed values
   vs the wanted values.
3. Extend the .info line to buildkernel/kernel-toolchain/toolchain/_cross-tools.
4. Consolidate all of the logic to one condition.

MFC after:	3 days
Sponsored by:	EMC / Isilon Storage Division

Makefile

@@ -131,7 +131,8 @@ TGTS=	all all-man buildenv buildenvvars buildkernel buildworld \
 	builddtb xdev xdev-build xdev-install \
 	xdev-links native-xtools stageworld stagekernel stage-packages \
 	create-world-packages create-kernel-packages create-packages \
-	packages installconfig real-packages sign-packages package-pkg
+	packages installconfig real-packages sign-packages package-pkg \
+	test-system-compiler
 
 # XXX: r156740: This can't work since bsd.subdir.mk is not included ever.
 # It will only work for SUBDIR_TARGETS in make.conf.
@@ -151,7 +152,8 @@ TGTS+=	${BITGTS}
 META_TGT_WHITELIST+= \
 	_* build32 buildfiles buildincludes buildkernel buildsoft \
 	buildworld everything kernel-toolchain kernel-toolchains kernel \
-	kernels libraries native-xtools showconfig tinderbox toolchain \
+	kernels libraries native-xtools showconfig test-system-compiler \
+	tinderbox toolchain \
 	toolchains universe world worlds xdev xdev-build
 
 .ORDER: buildworld installworld