target/arm: Implement FEAT_DoubleFault

The FEAT_DoubleFault extension adds the following:

 * All external aborts on instruction fetches and translation table
   walks for instruction fetches must be synchronous.  For QEMU this
   is already true.

 * SCR_EL3 has a new bit NMEA which disables the masking of SError
   interrupts by PSTATE.A when the SError interrupt is taken to EL3.
   For QEMU we only need to make the bit writable, because we have no
   sources of SError interrupts.

 * SCR_EL3 has a new bit EASE which causes synchronous external
   aborts taken to EL3 to be taken at the same entry point as SError.
   (Note that this does not mean that they are SErrors for purposes
   of PSTATE.A masking or that the syndrome register reports them as
   SErrors: it just means that the vector offset is different.)

 * The existing SCTLR_EL3.IESB has an effective value of 1 when
   SCR_EL3.NMEA is 1.  For QEMU this is a no-op because we don't need
   different behaviour based on IESB (we don't need to do anything to
   ensure that error exceptions are synchronized).

So for QEMU the things we need to change are:
 * Make SCR_EL3.{NMEA,EASE} writable
 * When taking a synchronous external abort at EL3, adjust the
   vector entry point if SCR_EL3.EASE is set
 * Advertise the feature in the ID registers

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20220531151431.949322-1-peter.maydell@linaro.org