AArch64: List arm64 as instruction set option

When arm64 was added to dex2oat, it wasn't added to the list of
instruction sets in the help text. This adds it.

Change-Id: I9ffecb67a053b24ad624effd1853ff47331d2e91
Signed-off-by: Stuart Monteith <stuart.monteith@arm.com>
diff --git a/dex2oat/dex2oat.cc b/dex2oat/dex2oat.cc
index c6e448e..ac76c35 100644
--- a/dex2oat/dex2oat.cc
+++ b/dex2oat/dex2oat.cc
@@ -139,7 +139,7 @@
   UsageError("      Example: --android-root=out/host/linux-x86");
   UsageError("      Default: $ANDROID_ROOT");
   UsageError("");
-  UsageError("  --instruction-set=(arm|mips|x86|x86_64): compile for a particular instruction");
+  UsageError("  --instruction-set=(arm|arm64|mips|x86|x86_64): compile for a particular instruction");
   UsageError("      set.");
   UsageError("      Example: --instruction-set=x86");
   UsageError("      Default: arm");