servo: Merge #9492 - Fix build error on AArch64 (from mmatyas:arm64_linker_fix); r=jdm

When cross compiling to AArch64, the native `cc` is used in the final linking step, instead of `aarch64-linux-gnu-gcc`. This patch fixes the issue.

Source-Repo: https://github.com/servo/servo
Source-Revision: ea385fdd117cf68e6550782371142a85a2a2af8a

servo/components/servo/.cargo/config

@@ -5,3 +5,7 @@ ar = "arm-linux-androideabi-ar"
 [target.arm-unknown-linux-gnueabihf]
 linker = "arm-linux-gnueabihf-gcc"
 ar = "arm-linux-gnueabihf-ar"
+
+[target.aarch64-unknown-linux-gnu]
+linker = "aarch64-linux-gnu-gcc"
+ar = "aarch64-linux-gnu-ar"