Use `rustc`'s knowledge of LLVM/Clang target triples #1961

Job	Run time
Test wasm32-wasip1-thread	33s
Test wasm (wasm32-unknown-unknown)	12s
Test CUDA support	50s
Test build-std (aarch64-tvos)	2m 11s
MSRV (ubuntu-latest)	25s
Test build-std (aarch64-tvos-sim)	1m 49s
MSRV (windows-latest)	49s
Test build-std (x86_64-tvos)	1m 55s
MSRV (macos-latest)	29s
Test (stable)	1m 15s
Test (beta)	1m 15s
Clippy	14s
Test (nightly)	1m 16s
Rustfmt	18s
Test (linux32)	2m 9s
Test (macos)	2m 39s
Test (aarch64-macos)	1m 26s
semver-checks	14s
Test (aarch64-ios)	49s
Test (win32)	1m 38s
Test (win64)	1m 44s
Test (mingw32)	2m 17s
Test (mingw64)	2m 38s
Test (windows-2019)	1m 41s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	4m 4s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	4m 26s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	4m 57s
	44m 13s

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