new(gnu.org/gcc/v8): GCC 8 for legacy C++ projects

[tannevaled]

Summary

• New recipe gnu.org/gcc/v8 — GCC 8.x line, parallel to the latest gnu.org/gcc recipe.
• Many projects vendoring older C++ libraries (RocksDB 6.3.x in cubefs, InnoDB in MySQL 5.7, pre-C++17 codebases) hit gcc 12+'s stricter copy-ctor / std::atomic / std::pair-construction rules. They compile cleanly with gcc 8 (last default-C++14 major).
• This recipe enables follow-ups like new(cubefs.io): CubeFS — cloud-native distributed storage (from source) #13050 (cubefs) and new(mysql.com/v5_7): MySQL 5.7 legacy (Top 300 #735) #13062 (mysql@5.7) to pin `gnu.org/gcc/v8: '*'` and compile their vendored old C++ from source — keeping the from-source promise without monkey-patching upstream sources.
• Linux only. Darwin's old-SDK situation makes gcc 8 hard to bring up, and llvm.org clang covers legacy on darwin.

Why

`pkgx +gnu.org/gcc^8` currently returns `ResolveError` — no 8.x bottle exists. This PR adds the recipe so pantry CI builds and bottles gcc 8 for downstream legacy recipes.

Test plan

• gcc 8.x builds on linux/x86-64 + linux/aarch64
• `gcc --version` reports "pkgx GCC 8.x.x"
• `gcc -dumpversion` starts with "8."
• Downstream test: cubefs.io can build RocksDB 6.3.6 with this gcc

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