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runner beyond bare metal: honour [target.<triple>].runner on hosted targets, typed launch failures, not-run reporting, [xlings] values per host (#544) #1517

Job	Run time
toolchain: gcc (cold self-host)	1m 55s
build + unit tests (linux x86_64, self-host)	1m 5s
integration: mcpp builds & runs xlings	1m 6s
toolchain: musl + llvm	1m 5s
	5m 11s