Fix distclean (mono/mono#16667)

Fix distclean (mono/mono#16667)

* Disable `subdir-objects` mode in Automake

This forces creation of subdirectories' files (e.g. .deps/*.Plo) in the
parent directory listing the source file. This works around a situation
where two different files specify files in a different parent directory
(i.e. ../../support/adler32.c in mono/metadata/ and adler32.c in
support/) both emit files into the same place (support/.deps), causing
`make distclean` to try and delete support/.deps from two different
places, breaking distclean.

This unfortunately adds a pile of Automake warnings, during autogen.

The "correct" fix is to stop specifying zlib as a pile of flat .c files,
and instead put it in its own subdirectory, conditionally including
its .la files as required.

Partially fixes https://github.com/mono/mono/issues/16605

* Ship netcore/Makefile with an empty distclean rule

We need a netcore/ directory because we write to it during configure,
and we need a netcore/Makefile with a distclean rule, so top-level
recursive distclean doesn't fail. So specify both here.

Partially fixes https://github.com/mono/mono/issues/16605

Commit migrated from https://github.com/mono/mono/commit/a67839540ef15948b4aebbdcee9ec44ee0f0e9a1