AArch64: Add arm64 runtime support.

Adds support for arm64 to ART. Assembler stubs are sufficient for
down calls into interpreter. JNI compiler and generics are not finished.

Basic Generic JNI functionality.

Change-Id: I4a07c79d1e037b9f5746673480e32cf456867b82
diff --git a/runtime/globals.h b/runtime/globals.h
index 5bc4b91..9c6fa0d 100644
--- a/runtime/globals.h
+++ b/runtime/globals.h
@@ -49,6 +49,10 @@
 // but ARM ELF requires 8..
 static constexpr size_t kArmAlignment = 8;
 
+// ARM64 instruction alignment. AArch64 require code to be 4-byte aligned.
+// AArch64 ELF requires at least 4.
+static constexpr size_t kArm64Alignment = 4;
+
 // MIPS instruction alignment.  MIPS processors require code to be 4-byte aligned.
 // TODO: Can this be 4?
 static constexpr size_t kMipsAlignment = 8;