Renamed x64 to x86

src/x86/REX.go

@@ -0,0 +1,7 @@
+package x86
+
+// REX is used to generate a REX prefix.
+// w, r, x and b can only be set to either 0 or 1.
+func REX(w, r, x, b byte) byte {
+	return 0b0100_0000 | (w << 3) | (r << 2) | (x << 1) | b
+}