Fix macos build by ensuring eigen index type uses int64_t

Previously we were drifting between LP64’s default (long) and a
hand-exported int64_t, which led to ABI/template mismatches
(“Eigen::Index == long” vs “int64_t”). This commit changes eigen's
default index type to int64_t to align with the reference models's
usage of eigen.

Change-Id: If88a6b137011c942044b6516e5a030e2db4bb92f
Signed-off-by: Luke Hutton <luke.hutton@arm.com>