Refine build system for efficient configurations

[jserv]

This addresses build system inefficiencies and compatibility issues:

1. Configuration change detection: Use cmp-based comparison to avoid unnecessary rebuilds when configuration hasn't actually changed. Previously, every make run would touch .config and trigger full rebuilds.
2. Directory creation optimization: Replace runtime mkdir -p $(shell dirname) calls with order-only prerequisites (|).
3. Configuration dependency tracking: Add $(CONFIG_FILE) as explicit prerequisite to all compilation rules. This ensures object files rebuild when feature flags change (e.g., ENABLE_JIT=1 -> 0).
4. Build system hygiene: Remove duplicate .PHONY declarations, fix shell script indentation in riscv-arch-test.mk, and add .PHONY coverage for all target declarations.

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Summary by cubic

Speeds up and stabilizes the Make build by only updating .config when values change and rebuilding objects when feature flags change. Also replaces ad‑hoc mkdir calls with proper directory prerequisites and adds clearer config warnings.

• Refactors

  • Update .config only when config actually changes; remove forced config step from default build.
  • Track $(CONFIG_FILE) in all compile rules (core, dev, tests, tools, softfloat) so files rebuild when flags change.
  • Use order-only prerequisites and explicit dir targets instead of inline mkdir.
  • Add GNU make version check for order-only prerequisites.
  • Clean up .PHONY targets and minor make/script formatting; soften noisy logs (use warnings where appropriate).
  • Add config validation messages (e.g., SDL with SYSTEM/FULL4G, T2C disabled note).

• Migration

  • Requires GNU make 3.81 or newer.