The targets seem to work as I would expect:
$ make check-stage2-cfail
cfg: build triple x86_64-apple-darwin
cfg: host triples x86_64-apple-darwin
cfg: target triples x86_64-apple-darwin
cfg: enabling more debugging (CFG_ENABLE_DEBUG)
cfg: host for x86_64-apple-darwin is x86_64
cfg: os for x86_64-apple-darwin is apple-darwin