Handle multi-valued `target_family` #350

Job	Run time
Rustfmt	7s
Cargo bench	6m 10s
Test (stable, x86_64-unknown-linux-gnu, ubuntu-20.04)	5m 10s
Test (stable, x86_64-fortanix-unknown-sgx, ubuntu-20.04)	2m 45s
Test (stable, x86_64-pc-windows-msvc, windows-latest)	15m 42s
Test (stable, x86_64-pc-windows-msvc, windows-2019)	14m 8s
Test (stable, aarch64-unknown-linux-musl, ubuntu-20.04)	7m 37s
Test (beta, x86_64-unknown-linux-gnu, ubuntu-20.04)	6m 38s
Test (nightly, x86_64-unknown-linux-gnu, ubuntu-20.04)	5m 57s
Test (stable, x86_64-apple-darwin, macos-latest)	5m 16s
ci	1s
	1h 9m 31s

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