Add arm64 source to linker_wrapper

As we enable arm64-based Bionic host target (linux_bionic_arm64),
linker_wrapper is added with the corresponding source.

Bug: 159685774
Test: HOST_CROSS_OS=linux_bionic HOST_CROSS_ARCH=arm64 m
Test: copy out/soong/host/linux_bionic_arm64/ to an ARM64 emulator
running Linux and execute the binaries

Change-Id: I4f367a349f7e0015318352cb7f2870fc856eab05
diff --git a/linker/Android.bp b/linker/Android.bp
index 08b2c7b..a51b73f 100644
--- a/linker/Android.bp
+++ b/linker/Android.bp
@@ -39,6 +39,9 @@
         "linker_wrapper.cpp",
     ],
     arch: {
+        arm64: {
+            srcs: ["arch/arm64/begin.S"],
+        },
         x86_64: {
             srcs: ["arch/x86_64/begin.S"],
         },