Use case

As a developer cross-compiling the flutter engine for a specific embedded platform, I want to specify the CPU model in order to get the best performance from the compiled code.

Proposal

Add a engine/src/flutter/tools/gn option --cpu-tune or similar that passes a specifc CPU variant through to the compiler's -mtune flag.

There is already support for this in the build via the arm_tune gn argument in arm.gni, for ARM CPUs. It just needs some support from the gn.py frontend.