Support resolving cross dependencies

[citrus-it]

Now that python 3.12 has been fixed so that it properly detects the
host triple, shared objects include this triple in the filename.
This means that the filename search list for a module called foo is
now:

        pkg.debug.depend.file=64/foo.abi3.so
        pkg.debug.depend.file=64/foo.cpython-312-x86_64-pc-solaris2.so
        pkg.debug.depend.file=64/foo.so
        pkg.debug.depend.file=foo.abi3.so
        pkg.debug.depend.file=foo.cpython-312-x86_64-pc-solaris2.so
        pkg.debug.depend.file=foo.py
        pkg.debug.depend.file=foo.pyc
        pkg.debug.depend.file=foo.pyo
        pkg.debug.depend.file=foo.so
        pkg.debug.depend.file=foo/__init__.py

This obviously doesn't work for aarch64, and we were effectively
getting away with it before because the shared objects were just named
foo.cpython-312.so. With this patch, if the environment contains

        PKG_CROSS_DEPEND=aarch64-unknown-solaris2

the search list is:

        pkg.debug.depend.file=foo.abi3.so
        pkg.debug.depend.file=foo.cpython-312-aarch64-unknown-solaris2.so
        pkg.debug.depend.file=foo.py
        pkg.debug.depend.file=foo.pyc
        pkg.debug.depend.file=foo.pyo
        pkg.debug.depend.file=foo.so
        pkg.debug.depend.file=foo/__init__.py

Note also that the 64/ paths have been removed for cross targets.