Add loongarch64 support

Author: zhaixiaojuan

src/lib.rs

@@ -2945,6 +2945,7 @@ impl Build {
             ]), // explicit None if not found, so caller knows to fall back
             "i686-unknown-linux-musl" => Some("musl"),
             "i686-unknown-netbsd" => Some("i486--netbsdelf"),
+            "loongarch64-unknown-linux-gnu" => Some("loongarch64-linux-gnu"),
             "mips-unknown-linux-gnu" => Some("mips-linux-gnu"),
             "mips-unknown-linux-musl" => Some("mips-linux-musl"),
             "mipsel-unknown-linux-gnu" => Some("mipsel-linux-gnu"),