Add GCC 15.3 toolchain

[nradakovic]

Summary

Introduce a new standalone GCC 15.3 Linux toolchain as a first-class, supported variant. The package bundles the full runtime/build stack:

• GCC 15.3 (compiler + all runtime libraries: libstdc++, libgcc, libgomp, etc. built against GCC 15.3)
• binutils 2.46.1 (ar, ld, strip, gcov, assembler, etc.)
• glibc 2.43
• Linux headers 7.0

All runtime libraries must be linked/connected to GCC 15.3 (no mixing with the existing 12.2.0 stack).

Motivation

The repository currently ships GCC 12.2.0 as the default Linux toolchain (x86_64-linux-gcc_12.2.0, aarch64-linux-gcc_12.2.0). Adding GCC 15.3 gives downstream consumers access to a newer compiler, C++ standard support, and updated runtime while keeping the existing version available side-by-side. Several critical parts are fixed within new toolchain version which will help developers to evaluate their code more efficiently.

Scope

• Create a standalone GCC 15.3 package (build descriptors + package metadata). (depends on score_toolchains_gcc_packages)
• Register it in the central version matrix.
• Wire it into the test workspace and validate via the existing feature/language suites.
• Add / extend CI so the new toolchain is exercised on pull requests.
• Update documentation.

Out of scope:

• removing or changing GCC 12.2.0, and QNX toolchains.
• adding support for aarch64-linux gcc 15.3

Acceptance Criteria (DoD)

• A standalone GCC 15.3 package resolves via the version matrix and produces a working C/C++ toolchain.
• All runtime libraries in the package are built against / linked to GCC 15.3.
• feature_verification_tests and language_and_standards_tests pass on the new x86_64-linux-gcc_15.3.0 config.
• CI runs and passes the new toolchain checks on pull requests.
• Existing GCC 12.2.0 toolchains continue to build and test unchanged (no regressions).
• Documentation reflects the new variant and its component versions.