X64 Crankshaft: Implement Math.abs on x64 lithium.

src/x64/macro-assembler-x64.cc

Index: src/x64/macro-assembler-x64.cc
===================================================================
--- src/x64/macro-assembler-x64.cc	(revision 6931)
+++ src/x64/macro-assembler-x64.cc	(working copy)
@@ -1503,6 +1503,11 @@
 }
 
 
+void MacroAssembler::LoadFromSafepointRegisterSlot(Register dst, Register src) {
+  movq(dst, SafepointRegisterSlot(src));
+}
+
+
 Operand MacroAssembler::SafepointRegisterSlot(Register reg) {
   return Operand(rsp, SafepointRegisterStackIndex(reg.code()) * kPointerSize);
 }