Change dSYM generation and native stripping to occur immediately before code signing,
in a newly minted post processing target.
Challenges:
* Both calling 'strip' and 'codesign' on an executable modifies that executable,
which means that we must make sure to not call 'dsymutil' on the same binary at
a later point unless it's been rebuilt.
* Thus we must make sure to update 'dsymutil's stamp file whenever we call 'strip'
and/or 'codesign' on an executable.
* Just like for code signing, we must store the libraries (either static or dynamic)
we post process in extension/watch/rid-specific projects, so that these libraries
can be loaded in containing projects and processed there.
* In universal .NET builds, debug symbols are created for the universal app bundle,
not for each rid-specific version of the app bundle. So I had to add logic to create
the native symbol lists (MtouchSymbolsList) for each rid-specific build, but then
collect them and merge those lists for the universal app bundle.
The existing SymbolStrip call we did right after linking the native executable has
been removed, because we have to do that after creating the dSYM (which the GenerateDebugSymbols
target does).
Also add tests.
Fixes https://github.com/xamarin/xamarin-macios/issues/14067.
e5c81564e2