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`<arch>-<os>` is a complete target on every platform (#500) #1316

Job	Run time
integration: mcpp builds & runs xlings	7m 29s
toolchain: gcc (cold self-host)	9m 17s
toolchain: musl + llvm	9m 47s
build + unit tests (linux x86_64, self-host)	13m 28s
	40m 1s