util: Force the m5 utility to be built using c++14.

util: Force the m5 utility to be built using c++14.

Otherwise, the version of the standard being used will depend on the
version of the compiler, and may not build consistently on different
machines.

Change-Id: I3a031a2dfe97df3dd83926abb517781069c1c6a5
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38216
Reviewed-by: Daniel Gerzhoy <daniel.gerzhoy@gmail.com>
Reviewed-by: Matt Sinclair <mattdsinclair@gmail.com>
Maintainer: Matt Sinclair <mattdsinclair@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>