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>

mem-cache: Proactive Instruction Fetch Implementation

Ferdman, M., Kaynak, C., & Falsafi, B. (2011, December).
Proactive instruction fetch. In Proceedings of the 44th Annual IEEE/ACM
International Symposium on Microarchitecture (pp. 152-162). ACM.

Change-Id: I38c3ab30a94ab279f03e3d5936ce8ed118310c0e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/16968
Reviewed-by: Nikos Nikoleris <nikos.nikoleris@arm.com>
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Nikos Nikoleris <nikos.nikoleris@arm.com>