First stage of dynamic recompiler rewrite.

Basic IR generation calls C implementation of all instructions.
Backend generation implemented for x86, x86-64, ARM and ARM64.

src/codegen_backend_x86_defs.h

@@ -0,0 +1,8 @@
+#define REG_EAX 0
+#define REG_ECX 1
+#define REG_EDX 2
+#define REG_EBX 3
+#define REG_ESP 4
+#define REG_EBP 5
+#define REG_ESI 6
+#define REG_EDI 7