Add GCC 15.3.0 toolchain

[4og]

This pull request adds support for GCC 15.3.0 toolchains for both x86_64 and aarch64 Linux builds, updates Bazel platform dependencies, and configures overrides to use the latest C++ toolchain sources. The main changes are grouped below:

• Added GCC 15.3.0 toolchains for both x86_64 and aarch64 Linux targets in MODULE.bazel, including new gcc.toolchain entries and their registration with use_repo.
• Introduced corresponding Bazel build and test configurations for these toolchains in .bazelrc, enabling builds and tests using GCC 15.3.0 for both architectures.

Note that the host toolchain remains GCC 12. It means that, for example, the flatbuffer's code generator (flatc) is built with GCC 12 even if the user passes --config=bl-x86_64-linux-gcc15.

[github-actions]

Documentation preview for this pull request is available at:
pr-488: https://eclipse-score.github.io/baselibs/pr-488/