Add plain cross-compilation mode for riscv64 and loongarch64

[kxxt]

This patch introduces a "plain" cross-compilation mode that cross compiles directly using cargo build and target-specific linker(gcc).

It can be used for architectures where cargo-zigbuild is broken(e.g. riscv64 #1793, loongarch64 #1645)

I have done test locally by running dist build -t riscv64gc-unknown-linux-gnu but not sure how to test such unreleased patches in CI context.

Close #1793