gcc-12: add gcc-12-default package which can provide the gcc slot

[kaniini]

Allow GCC 12 to act as system toolchain, instead of GCC 13, by providing the gcc name on a gcc-12-default package. This allows for GCC to be slotted like Java, Kubernetes, etc.

[github-actions]

Package gcc-12: Click to expand/collapse

Package gcc-12:
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Package gcc-12-doc: Click to expand/collapse

Package gcc-12-doc:
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Package libstdc++-12: Click to expand/collapse

Package libstdc++-12:
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Package libstdc++-12-dev: Click to expand/collapse

Package libstdc++-12-dev:
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Package gcc-12-default: Click to expand/collapse

Package gcc-12-default:
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