sys-kernel/tkg-sources: new package, add 6.12.7

[blackteahamburger]

之前linux-tkg-sources在#1295 被移除了，我认为有些欠考虑了：

• 用emerge比clone仓库之后再用install.sh方便
• 提供更多的灵活性，获取patch过的源码后就可以自行手工编译。而install.sh config存在一些问题（而且挺烦人）。
• 更完善的genpatches支持

pkgcheck的SizeViolation等Frogging-Family/linux-tkg#1058 解决

[github-actions]

⚪ pkgcheck checks passed but with something can be improved!

• pkgcheck 0.10.33
• the corresponding ::gentoo commit: gentoo-mirror/gentoo@ad3f997

QA Results:

sys-kernel/tkg-sources
  SizeViolation: 'files/6.12.7-0001-bore.patch' exceeds 20.0 KiB in size; 30.1 KiB total
  SizeViolation: 'files/6.12.7-more-ISA-levels-and-uarches-for-kernel-6.1.79+.patch' exceeds 20.0 KiB in size; 26.8 KiB total
  TotalSizeViolation: files/ directory exceeds 50.0 KiB in size; 56.9 KiB total

🔍 QA Keywords Explanation