.cargo: config: Enable riscv64 cross-compilation

Add linker for `--target=riscv64gc-unknown-linux-gnu`.

Signed-off-by: Tan En De <ende.tan@starfivetech.com>

Author: endeneer

.cargo/config

@@ -1,3 +1,5 @@
 [target.aarch64-unknown-linux-musl]
 rustflags = [ "-C", "target-feature=+crt-static", "-C", "link-arg=-lgcc" ]
 
+[target.riscv64gc-unknown-linux-gnu]
+linker = "riscv64-unknown-linux-gnu-gcc"