kernel kconfig: Do not enable STATIC_INIT_GNU w the old native_posix

native_posix (unlike native_sim) or its breathen (NATIVE_APPLICATION)
link together the "runner" code and the embedded code.
This means that when CONFIG_STATIC_INIT_GNU is set,
any host library code (like the llvm fuzzer) constructors will get
postponed to the Zephyr initialization.
These libraries are unlikely to work if we do this.
(the llvm fuzzer does not)

So let's instead not enable STATIC_INIT_GNU for these targets.
This means possible embedded library constructors will continue to be
picked during the link and be still called during the native runner
initialization instead of during the Zephyr OS initialization as they
were just before we introduced STATIC_INIT_GNU in
6e977ae2

Note that native_posix will be deprecated shortly and its users
are strongly encouraged to move to native_sim.

Signed-off-by: Alberto Escolar Piedras <alberto.escolar.piedras@nordicsemi.no>