arm: Fix maybe-uninitialized GCC warnings

arm: Fix maybe-uninitialized GCC warnings

GCC 7 generates maybe-uninitialized warnings at the code that updates
the "dest" variables in the writeVecElem function of neon64_mem.hh file.
It is because the generated code does not appropriately initialize the
output variable before passing it to the writeVecElem function.  This
patch initializes the output variable to fix this.

Change-Id: I50a8f4e456ccdcaa3db1392ec097017450c56ecb
Signed-off-by: Chun-Chen Hsu <chunchenhsu@google.com>
Reviewed-on: https://gem5-review.googlesource.com/9121
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>