Use riscv64-unknown-linux-gnu for target on clang. (#608)

Author: makotokato

cc-test/src/riscv64gc.S

@@ -0,0 +1,9 @@
+.globl asm
+asm:
+    li a0, 7
+    ret
+
+.globl _asm
+_asm:
+    li a0, 7
+    ret

src/lib.rs

@@ -1484,6 +1484,10 @@ impl Build {
                                 .into(),
                             );
                         }
+                    } else if target.starts_with("riscv64gc-") {
+                        cmd.args.push(
+                            format!("--target={}", target.replace("riscv64gc", "riscv64")).into(),
+                        );
                     } else {
                         cmd.args.push(format!("--target={}", target).into());
                     }