misc: Get gem5 to build with Clang 8

misc: Get gem5 to build with Clang 8

Added missing overrides:
- src/mem/token_port.hh
- src/sim/power/mathexpr_powermodel.hh

Remove Unused static constants:
- src/arch/x86/process.cc

Related Issue: https://gem5.atlassian.net/projects/GEM5/issues/GEM5-534

Change-Id: Icc725e2522dcee919e299f4ea7a9f1773f5dfa4d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/28947
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>