unittests/ASM: Test fdecstp and fincstp for C1 clear

unittests/ASM/FEX_bugs/X87ModifySTP_C1.asm

@@ -0,0 +1,31 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "RAX": "0x0000000000000000",
+    "RBX": "0x0000000000000000"
+  }
+}
+%endif
+
+
+fninit
+
+fldenv [rel env]
+fdecstp
+fnstsw ax
+and rax, 0x0200
+mov r8, rax
+
+fldenv [rel env]
+fincstp
+fnstsw ax
+and rax, 0x0200 
+mov rbx, rax
+mov rax, r8
+
+hlt
+
+env:
+    dd 0x037F ; FCW
+    dd 0x3200 ; FSW: C1=1
+    dd 0x0000 ; FTW