unittests/ASM: Test FINCSTP for tag word

unittests/ASM/FEX_bugs/fincstp_tag_word.asm

@@ -0,0 +1,24 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "RAX": "0xff"
+  }
+}
+%endif
+
+mov word [rel .fxsave_data + 0], 0x037f ; FCW
+mov byte [rel .fxsave_data + 4], 0xff   ; FTW
+
+; FSW = 0
+fxrstor [rel .fxsave_data]
+
+fincstp
+
+fxsave [rel .fxsave_data]
+movzx rax, byte [rel .fxsave_data + 4] ; FTW
+
+hlt
+
+align 4096
+.fxsave_data:
+times 64 dq 0