configure.py: add flag for riscv{64,32}gc musl-root

[jchzhou]

Add the corresponding flag for detecting --musl-root-riscv64gc and --musl-root-riscv32gc with ./configure, which is beneficial for downstream Linux distros to create the rust package with the same "recipe" from other architectures.

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[albertlarsan68]

LGTM
Thanks for the PR!
@bors r+ rollup

[bors]

📌 Commit d2516c1 has been approved by albertlarsan68

It is now in the queue for this repository.

[jyn514]

Why does this need its own flag? why not ./configure --set target.riscv64gc.musl-root=/foo?

[jchzhou]

Why does this need its own flag? why not ./configure --set target.riscv64gc.musl-root=/foo?

Good idea, thanks!
To implement that, though, I either need to change the original "recipe" from other architectures that --musl-root-ARCH works fine (which means extra steps of review by each affected arch's maintainers and days of overhead) or add an if on RISC-V, then do this way conditional, which is less elegant.

I see this commit as a kind of "compatibility" measure to make it "just works" upstream to avoid the aforementioned prolonged procedures and pains downstream.