new: add t3d.version file to embed in libdragon ROMs

[gobbledygoober]

This feature leverages @rasky's recent work on libdragon's preview branch towards embedding toolchain and libdragon version information into ROMs.

This change (shamelessly) lifts the supporting Makefile logic and comments from libdragon, and generates a t3d.version file that will be picked up by N64_TOOLFILES in n64.mk.

Tested by regenerating 99_testscene (see new t3d-* lines):

strings t3d_99_testscene.z64 | tail -n 11                  
  "branch": "preview",
  "hash": "06bf1cb00d384f93d5d61fca386ecc5a96ed391e",
  "commit-date": "2025-11-09",                   
  "dirty": true
  "t3d-hash": "db87c29baa6d8688c12fd51f18e84da0455059f3",
  "t3d-commit-date": "2025-11-10",               
  "t3d-dirty": false
  "host": "x86_64-pc-linux-gnu",                 
  "binutils": "2.44",
  "gcc": "14.2.0",
  "newlib": "4.4.0.20231231"               

I mostly made this for selfish reasons, to confirm my understand of the mechanisms involved. So feel free to close or delay work on this if it's premature. Thanks for reading!