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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) #1518

Job	Run time
integration: mcpp builds & runs xlings	9m 25s
toolchain: gcc (cold self-host)	10m 6s
toolchain: musl + llvm	10m 33s
build + unit tests (linux x86_64, self-host)	16m 16s
	46m 20s