unittests/ASM: Test FST ST(0) tag word update

unittests/ASM/FEX_bugs/fst_st0_tag_word.asm

@@ -0,0 +1,27 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "RAX": "0x1"
+  }
+}
+%endif
+
+
+fxrstor [rel .fxsave_data]
+fst st0
+
+fxsave [rel .save]
+movzx eax, byte [rel .save + 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