Intrinsify System.arraycopy.

Currently on x64, will do the other architectures in
different changes.

Change-Id: I15fbbadb450dd21787809759a8b14b21b1e42624
diff --git a/compiler/optimizing/code_generator_x86_64.h b/compiler/optimizing/code_generator_x86_64.h
index ecc8630..d6a6a7e 100644
--- a/compiler/optimizing/code_generator_x86_64.h
+++ b/compiler/optimizing/code_generator_x86_64.h
@@ -30,6 +30,9 @@
 // Use a local definition to prevent copying mistakes.
 static constexpr size_t kX86_64WordSize = kX86_64PointerSize;
 
+// Some x86_64 instructions require a register to be available as temp.
+static constexpr Register TMP = R11;
+
 static constexpr Register kParameterCoreRegisters[] = { RSI, RDX, RCX, R8, R9 };
 static constexpr FloatRegister kParameterFloatRegisters[] =
     { XMM0, XMM1, XMM2, XMM3, XMM4, XMM5, XMM6, XMM7 };