unittests: Adds a 3DNow! ModRM SIB encoding test

Depends on #4239.

This codepath was unttested in our CI.

unittests/ASM/FEX_bugs/3DNow_ModRMSIBDecode.asm

@@ -0,0 +1,24 @@
+%ifdef CONFIG
+{
+  "RegData": {
+    "MM0":  "0x3f800000bf800000"
+  },
+  "HostFeatures": ["3DNOW"]
+}
+%endif
+
+; FEX-Emu had a bug with 3DNow! ModRM decoding when the source was SIB encoded.
+; This would result in a crash in the frontend instruction decoding.
+; Generate a 3DNow! instruction that uses SIB encoding to ensure this code path is tested.
+lea rax, [rel data1]
+mov rbx, 0
+pi2fw mm0, [rbx * 8 + rax + 0]
+
+hlt
+
+align 8
+data1:
+dw -1
+dw 0xFF
+dw 1
+dw 0xFF