unittests/ASM: Test FXCH for tag word

unittests/ASM/FEX_bugs/fxch_tag_word.asm

@@ -0,0 +1,40 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "RAX": "0x3",
+    "RBX": "0x81"
+  }
+}
+%endif
+
+; FXCH: empty and empty operands -> mark both valid
+fxrstor [rel .fxsave_data]
+fxch st0, st1
+
+fxsave [rel .save]
+movzx eax, byte [rel .save + 4]
+
+; FXCH: valid and empty operands -> mark both valid
+finit
+fldz
+fxch st0, st1
+
+fxsave [rel .save2]
+movzx ebx, byte [rel .save2 + 4]
+
+hlt
+
+align 16
+.fxsave_data:
+dw 0x037f       ; FCW
+dw 0x0000       ; FSW
+dw 0x0000       ; FTW
+times 506 db 0
+
+align 16
+.save:
+times 64 dq 0
+
+align 16
+.save2:
+times 64 dq 0