arch-arm: Fix FSC generation in AbortFault

arch-arm: Fix FSC generation in AbortFault

The fault status code generated by a Prefetch/Data Fault was containing
a wrong value when the fault was triggered in aarch32 but handled in
aarch64.  This because the encoding differs between the two ISAs and the
encoder was just checking the starting ISA rather than the the ending
one. In this case the getFsr must be called after we know which is the
ending ISA, which happens only after ArmFault::invoke gets called.  The
fsc update hence happens before writing into the Syndrome register.

Change-Id: I725f12b6dcc0178f608233bd3d15e466d1cd1ffc
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/8362
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>