target/arm: Add assertion about FSC format for syndrome registers

In tlb_fill() we construct a syndrome register value from a
fault status register value which is filled in by arm_tlb_fill().
arm_tlb_fill() returns FSR values which might be in the format
used with short-format page descriptors, or the format used
with long-format (LPAE) descriptors. The syndrome register
always uses LPAE-format FSR status codes.

It isn't actually possible to end up delivering a syndrome
register value to the guest for a fault which is reported
with a short-format FSR (that kind of stage 1 fault will only
happen for an AArch32 translation regime which doesn't have
a syndrome register, and can never be redirected to an AArch64
or Hyp exception level). Add an assertion which checks this,
and adjust the code so that we construct a syndrome with
an invalid status code, rather than allowing set bits in
the FSR input to randomly corrupt other fields in the syndrome.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Edgar E. Iglesias <edgar.iglesias@xilinx.com>
Message-id: 1491486152-24304-1-git-send-email-peter.maydell@linaro.org