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/assembler.cc b/compiler/utils/assembler.cc
index 6732476..a7cb278 100644
--- a/compiler/utils/assembler.cc
+++ b/compiler/utils/assembler.cc
@@ -111,7 +111,8 @@
       return new arm64::Arm64Assembler();
     case kMips:
       return new mips::MipsAssembler();
-    case kX86:
+    case kX86:  // Fall-through.
+    case kX86_64:
       return new x86::X86Assembler();
     default:
       LOG(FATAL) << "Unknown InstructionSet: " << instruction_set;