Replace future deprecated call

The trim_right_matches method is deprecating in 1.33 in favor of
trim_end_matches (see rust-lang/rust#52994).

src/lib.rs

@@ -1719,7 +1719,7 @@ impl Build {
                 } else if self.get_host()? != target {
                     // CROSS_COMPILE is of the form: "arm-linux-gnueabi-"
                     let cc_env = self.getenv("CROSS_COMPILE");
-                    let cross_compile = cc_env.as_ref().map(|s| s.trim_right_matches('-'));
+                    let cross_compile = cc_env.as_ref().map(|s| s.trim_end_matches('-'));
                     let prefix = cross_compile.or(match &target[..] {
                         "aarch64-unknown-linux-gnu" => Some("aarch64-linux-gnu"),
                         "aarch64-unknown-linux-musl" => Some("aarch64-linux-musl"),