Add entry for riscv64gc-unknown-netbsd (to support NetBSD/riscv64). (… #371

Job	Run time
Rustfmt	10s
Test (x86_64-unknown-linux-gnu, stable, ubuntu-latest)	6m 31s
Test (x86_64-unknown-linux-gnu, beta, ubuntu-latest)	6m 39s
Test (x86_64-unknown-linux-gnu, nightly, ubuntu-latest)	8m 31s
Test (x86_64-apple-darwin, stable, macos-latest)	6m 43s
Test (aarch64-apple-darwin, nightly, macos-latest)	5m 47s
Test (i686-unknown-linux-gnu, stable, ubuntu-latest)	3m 49s
Test (arm-unknown-linux-gnueabi, stable, ubuntu-latest)	4m 50s
Test (aarch64-unknown-linux-gnu, stable, ubuntu-latest)	4m 37s
Test (x86_64-unknown-linux-musl, stable, ubuntu-latest)	4m 9s
Test (x86_64-pc-windows-gnu, stable, ubuntu-latest)	4m 55s
Test (wasm32-wasi, stable, ubuntu-latest)	5m 22s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019)	17m 56s
Test (i686-pc-windows-msvc, stable-i686-msvc, windows-2019, yes)	18m 16s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest)	23m 29s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, yes)	22m 24s
Test (x86_64-pc-windows-msvc, stable-x86_64-msvc, windows-latest, installed)	25m 18s
	2h 49m 26s

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