Build fails with GCC 15+ due to `__STDC_EMBED_*` macro redefinitions

[tarun-t]

F-Stack's mk/kern.pre.mk dumps all predefined macros into filtered_predefined_macros.h but doesn't filter __STDC_EMBED_EMPTY__, __STDC_EMBED_FOUND__, and __STDC_EMBED_NOT_FOUND__. GCC 15 introduced these as built-in macros (C23 #embed), causing -Werror redefinition errors.

Fix: add these to IMACROS_FILTER in mk/kern.pre.mk:

IMACROS_FILTER+= STDC_EMBED_EMPTY STDC_EMBED_FOUND STDC_EMBED_NOT_FOUND