base: Use a const auto & in a range based for loop.

base: Use a const auto & in a range based for loop.

clang 11 complains otherwise which breaks the build, and this way
is less verbose and more efficient since it doesn't require copying
a vector of strings for each element of the loop.

Change-Id: I005fa5fdf19ddba2114e98413e3609b0a91c1ec5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25226
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>