arch-x86: Don't load past the end of the far pointer in real mode jmp.

When loading the segment selector from the far pointer, only load two
bytes to avoid unnecessarily going beyond a boundary where accesses may
not be allowed.

Change-Id: I4fc31e3f87815a19232390966c25d156be6a7e92
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/55624
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/isa/insts/general_purpose/control_transfer/jump.py

@@ -151,7 +151,7 @@ def macroop JMP_FAR_REAL_M
     .control_indirect
 
     lea t1, seg, sib, disp, dataSize=asz
-    ld t2, seg, [1, t0, t1], dsz
+    ld t2, seg, [1, t0, t1], dsz, dataSize=2
     ld t1, seg, [1, t0, t1]
     zexti t3, t2, 15, dataSize=8
     slli t3, t3, 4, dataSize=8