risc-v: Use medany model (#467)

With rust-lang/rust#62281, rust compiles in mcmodel=medium mode
which is equivalent to GCC medany. This prevents linker relocation
errors code is placed outside the range `-0x80000000..0x7ffffffff`.

src/lib.rs

@@ -1603,6 +1603,7 @@ impl Build {
                         } else {
                             cmd.args.push("-mabi=ilp32".into());
                         }
+                        cmd.args.push("-mcmodel=medany".into());
                     }
                 }
             }