add env var to override active eth kernel compiler opt-level

[SeanNijjar]

What's changed

Adding a temporary option to let a user override active eth kernel compiler opt-level. Let Llama demo compile at -O3 before official per-kernel opt-level overrides are enabled.

Specifying an opt-level:
TT_METAL_ACTIVE_ETH_OPT_LEVEL_OVERRIDE=O3
Valid overrides are "O0", "Os", "O1", "O2", "O3" -> omit the '-'.

If invalid opt-level specified:
Invalid opt level override for 'TT_METAL_ACTIVE_ETH_OPT_LEVEL_OVERRIDE'. Valid strings are "O0", "Os", "O1", "O2", and "O3" but provided string is 'O4'.

quick sweep of 4k packet size 1D fabric mcast test shows the following cycle counts across opt-levels:

-O0: Binary too big
-Os: 283106502
-O1: 285658208
-O2: 249658742
-O3: 251982726

A ~13% improvement is seen when moving from -Os to either -O2 or -O3

Checklist

• All post commit CI: https://github.com/tenstorrent/tt-metal/actions/runs/13507650581
• Blackhole Post commit CI: https://github.com/tenstorrent/tt-metal/actions/runs/13507662512
• Model regression CI passes (if applicable)
• Device performance regression CI passes (if applicable)
• (For models and ops writers) Full new models tests CI passes (if applicable)
• New/Existing tests provide coverage for changes