Use `rustc`'s knowledge of LLVM/Clang target triples #1957

Job	Run time
Test wasm32-wasip1-thread	33s
Test build-std (aarch64-tvos)	1m 41s
Test build-std (aarch64-tvos-sim)	2m 50s
Clippy	16s
Test wasm (wasm32-unknown-unknown)	15s
Test build-std (x86_64-tvos)	1m 45s
Test CUDA support	44s
MSRV (ubuntu-latest)	22s
Rustfmt	15s
MSRV (windows-latest)	46s
semver-checks	17s
MSRV (macos-latest)	46s
Test (stable)	1m 19s
Test (beta)	1m 39s
Test (nightly)	1m 23s
Test (linux32)	2m 10s
Test (macos)	4m 48s
Test (aarch64-macos)	2m 37s
Test (aarch64-ios)	1m 41s
Test (win32)	1m 59s
Test (win64)	2m 56s
Test (mingw32)	2m 11s
Test (mingw64)	2m 41s
Test (windows-2019)	3m 1s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 53s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 27s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 28s
	49m 43s

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