Rework riscv -march and -mabi detection #1632

Job	Run time
Test CUDA support	33s
Clippy	9s
MSRV (ubuntu-latest)	24s
Test build-std (aarch64-tvos)	1m 49s
MSRV (windows-latest)	40s
Test build-std (aarch64-tvos-sim)	1m 30s
Rustfmt	8s
semver-checks	2m 19s
Test build-std (x86_64-tvos)	1m 41s
Test (stable)	1m 1s
Test (beta)	1m 4s
Test (nightly)	1m 15s
Test (linux32)	1m 18s
Test (macos)	2m 30s
Test (aarch64-macos)	1m 18s
Test (aarch64-ios)	1m 26s
Test (win32)	2m 33s
Test (win64)	3m 3s
Test (mingw32)	2m 19s
Test (mingw64)	2m 19s
Test (windows-2019)	2m 40s
Test (windows-aarch64, windows-latest, stable, aarch64-pc-windows-msvc, --no-run)	3m 15s
Test (windows-clang, windows-2019, stable, x86_64-pc-windows-msvc, clang, clang++)	3m 28s
Test (windows-clang-cl, windows-2019, stable, x86_64-pc-windows-msvc, clang-cl, clang-cl)	3m 31s
	42m 13s

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