RTEMS Source Builder | config: Move GCC 15 and HEAD to their own backend files (!253)

[Kinsey Moore (@opticron)]

Kinsey Moore created a merge request: https://gitlab.rtems.org/rtems/tools/rtems-source-builder/-/merge_requests/253

Project:Branches: opticron/rtems-source-builder:brmain-separate-gcc-backends to rtems/tools/rtems-source-builder:main
Author:   Kinsey Moore
Assignee: Kinsey Moore
Reviewer: Chris Johns

## Summary

config: Move GCC 15 and HEAD to their own backend files

This moves GCC 15 and HEAD configurations to their own backend files so
that they can be managed independently without affecting other versions.

Closes #177

I tested this by building 7/rtems-aarch64 (GCC 15.2) and edge/rtems-arm (GCC HEAD/16). When we bring 16 into next, we can copy gcc-head.cfg to gcc-16.cfg.

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