Make our architecture defines less stupid.

Our defines were never clear between what meant 64bit or x86_64
This makes a clear cut between bitness and architecture.
This commit also has the side effect of bringing up aarch64 compiling support.

Source/Core/Core/DSP/DSPEmitter.cpp

@@ -401,7 +401,7 @@ void DSPEmitter::CompileDispatcher()
 
 
 	// Execute block. Cycles executed returned in EAX.
-#ifdef _M_IX86
+#if _M_X86_32
 	MOVZX(32, 16, ECX, M(&g_dsp.pc));
 	MOV(32, R(EBX), ImmPtr(blocks));
 	JMPptr(MComplex(EBX, ECX, SCALE_4, 0));