Fix host architecture for 64bit.

Also, hack x86 assembler for use as a x86-64 trampoline compiler's assembler.
Implement missing x86-64 quick resolution trampoline.
Add x86-64 to the quick elf writer.

Change-Id: I08216c67014a83492ada12898ab8000218ba7bb4
diff --git a/compiler/utils/x86/assembler_x86.cc b/compiler/utils/x86/assembler_x86.cc
index 136d248..26300e0 100644
--- a/compiler/utils/x86/assembler_x86.cc
+++ b/compiler/utils/x86/assembler_x86.cc
@@ -1210,6 +1210,13 @@
   return this;
 }
 
+X86Assembler* X86Assembler::gs() {
+  // TODO: fs is a prefix and not an instruction
+  AssemblerBuffer::EnsureCapacity ensured(&buffer_);
+  EmitUint8(0x65);
+  return this;
+}
+
 void X86Assembler::AddImmediate(Register reg, const Immediate& imm) {
   int value = imm.value();
   if (value > 0) {