Add support for riscv32-unknown-linux-gnu/musl targets

[newpavlov]

Checklist

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Describe your request

Support for running tests on 32-bit RISC-V targets similar to the existing riscv64-unknown-linux-gnu target support.

Describe why this would be a good inclusion for cross

It would be really nice to be able to test code on both 32 and 64 bit RISC-V targets. QEMU already supports 32 bit targets, so it should not be a blocker.