kernel: Unify .ctors and .init_array handling

Handle both of these sections in a single chunk of code instead of
separately. We don't need to use the legacy .ctors ABI as both
the constructors array and startup logic are managed within a single
link result.

This can now also be used with ARC MWDT which had been using the .ctors
sections but with .init_array semantics. For ARC MWDT, we now always
discard .dtors and .fini sections as Zephyr will never cause global
destructors to execute. Stop discarding .eh_frame sections so that
exception handling works as expected.

When building a NATIVE_APPLICATION, we ask the native C library to run all
of the constructors to ensure any non-Zephyr constructors are run before
main is invoked. It might be "nice" to split the constructors so that the
Zephyr constructors were executed by the Zephyr code while the non-Zephyr
ones were executed by the native C library. I think that could be done if
we knew the pathnames of either the Zephyr or non-Zephyr files. That might
make a good future enhancement.

Signed-off-by: Keith Packard <keithp@keithp.com>