[bk72xx] Emit USE_BK72XX_CRASH_HANDLER via cg.add_define

The previous gate lived only in core/defines.h, which is loaded for
static analysis / IDE support but not for actual builds. As a result
the wrap symbols (__wrap_bk_trap_udef/pabt/dabt) were preprocessed out
of crash_handler.cpp at compile time, leaving the linker without a
target for the SDK's wrapped trap calls. Set the define from
libretiny/__init__.py (where the platform-specific codegen lives —
bk72xx/__init__.py is auto-generated and not the right home).

esphome/components/libretiny/__init__.py

@@ -486,6 +486,10 @@ async def component_to_code(config):
         # a .noinit region before the SDK's own register dump + bk_cpu_shutdown
         # chain runs. The .noinit region is injected into the generated linker
         # script by patch_bk72xx_noinit.py so it survives the watchdog reset.
+        # bk72xx/__init__.py is auto-generated, so the define is set here
+        # rather than there. (defines.h has the matching #ifdef USE_BK72XX
+        # block for static analysis / IDE only — it doesn't drive the build.)
+        cg.add_define("USE_BK72XX_CRASH_HANDLER")
         for sym in ("bk_trap_udef", "bk_trap_pabt", "bk_trap_dabt"):
             cg.add_build_flag(f"-Wl,--wrap={sym}")
         cg.add_platformio_option("extra_scripts", ["pre:patch_bk72xx_noinit.py"])