arch-x86: Handle a special case for MODRM in 16 bit mode.

When the address size is 16 bit, the mod field is 0, and the rm is 6,
there is no base register, only a displacement.

Change-Id: Ib67a6e5ce617d08913b9ca6dee66877f0154ffe1
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55285
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/x86/emulenv.cc

@@ -82,7 +82,8 @@ void EmulEnv::doModRM(const ExtMachInst & machInst)
                     base = INTREG_RDI;
                     break;
                   case 6:
-                    base = INTREG_RBP;
+                    // There is a special case when mod is 0 and rm is 6.
+                    base = machInst.modRM.mod == 0 ? INTREG_T0 : INTREG_RBP;
                     break;
                   case 7:
                     base = INTREG_RBX;