Fix instrumentation exit stub for arm64

Fixes bad offset to restore register x21.

Bug: 20798393

(cherry picked from commit a538effb3f848bfec232c55f167b9f35b416948f)

Change-Id: I030d848ab0f46043e86f53abae204463f0aaccbe
diff --git a/runtime/arch/arm64/quick_entrypoints_arm64.S b/runtime/arch/arm64/quick_entrypoints_arm64.S
index 1d316fc..f8b0734 100644
--- a/runtime/arch/arm64/quick_entrypoints_arm64.S
+++ b/runtime/arch/arm64/quick_entrypoints_arm64.S
@@ -182,7 +182,7 @@
     // Restore xSELF as it might be scratched.
     mov xSELF, xETR
     // ETR
-    ldr xETR, [sp, #16]
+    ldr xETR, [sp, #32]
     .cfi_restore x21
 
     add sp, sp, #112