llama-gemma3-cli: Sigint rework in gemma3 vision example

[pl752]

Purpose
Converted SIGINT handler to notify the main through the flag, instead of just exiting
Added the logic to interrupt the program in the predictable states

Rationale
Original code abruptly ends the program on sigint, without flushing stdout or printing any stats
Changed the behavior to only setting global state flags or force quit, if the application didn't respond to the flag change
State flags are volatile, due to them being changed in the interrupt and only read in the main code, which doesn't guarantee proper handling of unobserved changes in some of the compilers with enabled -O2 or higher.

[ngxson]

There is an on-going work and llama-gemma3-cli will be soon moved to llama-mtmd-cli, I think we should go back to this PR later because this is not very important atm

See: #13012

[pl752]

Okay, thank you for the response