Enhance target architecture selection rules.

Change-Id: I595f8b6d9acd36bffc0017f06a959e709b9707d5
diff --git a/lib/Disassembler/Disassembler.cpp b/lib/Disassembler/Disassembler.cpp
index 8a2cf2c..d0e6000 100644
--- a/lib/Disassembler/Disassembler.cpp
+++ b/lib/Disassembler/Disassembler.cpp
@@ -67,12 +67,11 @@
 namespace bcc {
 
 void InitializeDisassembler() {
-#if defined(DEFAULT_ARM_CODEGEN) || defined(PROVIDE_ARM_CODEGEN)
+#if defined(PROVIDE_ARM_CODEGEN)
   LLVMInitializeARMDisassembler();
 #endif
 
-#if defined(DEFAULT_X86_CODEGEN) || defined(PROVIDE_X86_CODEGEN) || \
-    defined(DEFAULT_X64_CODEGEN) || defined(PROVIDE_X64_CODEGEN)
+#if defined(PROVIDE_X86_CODEGEN)
   LLVMInitializeX86Disassembler();
 #endif
 }