Replace $(shell) with portable $(python)

[jserv]

This migrates all $(shell,...) preprocessor calls in the toolchain detection section to portable $(python,...) built-in from Kconfiglib, eliminating POSIX shell dependency (2>/dev/null, ||, &&) and enables Windows-compatible builds.

• CC_IS_EMCC/CLANG/GCC: use run() with sys.executable for shell-free subprocess execution under the same Python interpreter
• COMPILER_TYPE: derive from CC_IS_* booleans via conditional defaults, eliminating a redundant subprocess call entirely
• HAVE_EMCC: use in-process shutil.which() with no subprocess
• HAVE_SDL2: guard with shutil.which('pkg-config') to silently return n when pkg-config is absent, then run() for the check

---

Summary by cubic

Replaced all $(shell,...) calls in toolchain detection with Kconfiglib’s $(python,...) to remove the POSIX shell dependency and enable Windows builds. Simplified compiler type and dependency checks.

• Refactors
  • CC_IS_EMCC/CLANG/GCC use run(sys.executable, ...) for shell-free checks.
  • COMPILER_TYPE is derived from CC_IS_* via conditional defaults.
  • HAVE_EMCC uses shutil.which('emcc'); HAVE_SDL2 guards with shutil.which('pkg-config') then runs the pkg-config check.

^{Written for commit 8615a31. Summary will update on new commits.}

[cubic-dev-ai]

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