target-i386: fix segment limit check in ljmp

target-i386: fix segment limit check in ljmp

The current implementation has three bugs,
 * segment limits are not enforced in protected mode if the L bit is set
   in the target segment descriptor
 * segment limits are not enforced in compatibility mode (ljmp to 32-bit
   code segment in long mode)
 * #GP(new_cs) is generated rather than #GP(0)

Now the segment limits are enforced if we're not in long mode OR the
target code segment doesn't have the L bit set.

Signed-off-by: Andrew Oates <aoates@google.com>
Message-Id: <20180816011903.39816-1-andrew@andrewoates.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>