Implement explicit definitions for required legacy Bazel C++ features

[nradakovic]

What

Implement explicit toolchain feature definitions for every required behavior that is currently obtained implicitly from Bazel's legacy C++ feature set.

The goal is to make the toolchain configuration explicit, deterministic, and independent from Bazel's built-in legacy defaults.

Acceptance Criteria (DoD)

• All required features identified during gap analysis are implemented explicitly in the toolchain configuration, or intentionally marked unsupported.
• Supported Linux and QNX toolchain behavior no longer depends on Bazel implicit legacy features.
• The explicit feature set is traceable back to the inventory and gap-analysis outputs.
• The toolchain continues to support all intended build scenarios after the change.

How

• Use the gap-analysis output as the implementation backlog.
• Add explicit feature definitions to the relevant toolchain templates.
• Keep Linux- and QNX-specific behavior separate where platform requirements differ.
• Preserve supported behavior where that behavior is intended.
• Where a legacy behavior is no longer desired, make that decision explicit rather than silently relying on Bazel defaults.
• Keep changes scoped feature by feature so review and validation remain manageable.

Out of Scope

• Comprehensive consumer documentation.
• CI expansion not required to validate the implementation itself.
• Long-term regression guardrails.