arm: Use zero initialization for the BigRegVect types.

arm: Use zero initialization for the BigRegVect types.

These were being initialized with BigRegVect brv = {0}, which made the
compiler complain because there is internal structure. The first element
of the union is actually an array, and this was telling it to initialize
that array to scalar 0. It was warning about this which was breaking the
build.

Instead, use zero initlization like BigRegVect brv = {}. This
initializes the first element of the union to all zeroes, with all
padding bits initialized to zero as well.

This satisfies the compiler and avoids a build error.

Change-Id: I31e7a8730c538637ff2e0c7fb00a4e12ed05e074
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34575
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>