Compiler abstraction

pbl/kernel/compiler.h is the only place the tree may spell compiler specifics: attributes (PBL_PACKED, PBL_WEAK, PBL_NAKED, PBL_SECTION(), …) and builtins (PBL_LIKELY(), PBL_RETURN_ADDRESS(), PBL_CLZ(), …). It only covers what C23 does not: use [[noreturn]], [[maybe_unused]], [[deprecated]], unreachable() and ckd_add()/ckd_mul() directly. Every public macro is declared and documented once in the frontend and expands to a *_IMPL counterpart from the backend selected by the predefined macros: compiler/gcc.h for GCC and compiler/clang.h for Clang, which reuses the GCC definitions and blanks the attributes Clang does not implement. Supporting another compiler means adding a backend that defines the same *_IMPL set and a branch in the frontend; callers stay untouched. The unit-test hooks built on these (PBL_T_STATIC, PBL_T_MOCKABLE) live in pbl/util/testing.h. Code outside include/pbl/kernel/compiler/ must not use __attribute__ or __builtin_* directly. Exported headers follow the same rule: the SDK generator rewrites the macros into their GCC spellings, so the SDK ships no firmware headers; see Building the SDK.