AArch64: Add ARM64 Assembler

This patch adds the ARM64 Assembler and ManagedRegister backend.
The implementation of the Arm64Assembler class is based on VIXL (a
programmatic A64 Assembler - see external/vixl ).

Change-Id: I842fd574637a953c19631eedf26f6c70d9ed7f9e
Signed-off-by: Serban Constantinescu <serban.constantinescu@arm.com>
diff --git a/runtime/instruction_set.h b/runtime/instruction_set.h
index ac83601..cbc9912 100644
--- a/runtime/instruction_set.h
+++ b/runtime/instruction_set.h
@@ -27,6 +27,7 @@
 enum InstructionSet {
   kNone,
   kArm,
+  kArm64,
   kThumb2,
   kX86,
   kX86_64,