mem-cache: ambiguous use of abs function

mem-cache: ambiguous use of abs function

std::abs doesn't accept unsigned long long, generating the error:

error: call to 'abs' is ambiguous

Use instead a compare-and-subtract idiom.

Also, Changed return type of distanceFromTrigger from unsigned int to Addr to
prevent overflow problems.

Change-Id: Ia7752c1c7a838f98e8c7ed6ade9f586f31bbcf7d
Signed-off-by: Ryan Gambord <gambordr@oregonstate.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/17788
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>