Handle multi-valued `target_family` #347

Job	Run time
Rustfmt	11s
Test (stable, x86_64-unknown-linux-gnu, ubuntu-20.04)	5m 15s
Cargo bench	6m 6s
Test (stable, x86_64-fortanix-unknown-sgx, ubuntu-20.04)	3m 22s
Test (stable, x86_64-pc-windows-msvc, windows-latest)	18m 41s
Test (stable, x86_64-pc-windows-msvc, windows-2019)	16m 40s
Test (stable, aarch64-unknown-linux-musl, ubuntu-20.04)	7m 39s
Test (beta, x86_64-unknown-linux-gnu, ubuntu-20.04)	7m 41s
Test (nightly, x86_64-unknown-linux-gnu, ubuntu-20.04)	7m 7s
Test (stable, x86_64-apple-darwin, macos-latest)	6m 4s
ci	1s
	1h 18m 47s

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