unittests: Add test for x87 mode switches wrongly flushing NZCV

unittests/ASM/FEX_bugs/X87MMXNZCV.asm

@@ -0,0 +1,13 @@
+%ifdef CONFIG
+{
+  "RegData": {},
+  "Env": { "FEX_X87REDUCEDPRECISION" : "1" }
+}
+%endif
+
+; FEX had a bug where a mmx->x87 switch would flush the saved NZCV value used for ftst, causing a crash in RA
+
+movq mm0, mm1 ; enters mmx state
+ftst ; enters x87 state
+
+hlt