Support riscv64gc-unknown-linux-musl #389

Job	Run time
Rustfmt	7s
Test (x86_64-unknown-linux-gnu, stable, ubuntu-latest)	8m 50s
Test (x86_64-unknown-linux-gnu, beta, ubuntu-latest)	8m 16s
Test (x86_64-unknown-linux-gnu, nightly, ubuntu-latest)	6m 31s
Test (x86_64-apple-darwin, stable, macos-latest)	10m 37s
Test (aarch64-apple-darwin, nightly, macos-latest)	7m 34s
Test (i686-unknown-linux-gnu, stable, ubuntu-latest)	3m 41s
Test (arm-unknown-linux-gnueabi, stable, ubuntu-latest)	3m 59s
Test (aarch64-unknown-linux-gnu, stable, ubuntu-latest)	4m 2s
Test (x86_64-unknown-linux-musl, stable, ubuntu-latest)	4m 23s
Test (x86_64-pc-windows-gnu, stable, ubuntu-latest)	5m 35s
Test (wasm32-wasi, stable, ubuntu-latest)	6m 11s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019)	21m 19s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019, yes)	18m 23s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest)	23m 8s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, yes)	14m 58s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, installed)	1m 25s
	2h 28m 59s

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