loader: fix elf lookup_symbol type filtering

The existing logic doesn't seem to make much sense, as we won't filter
on the type if st_shndx != SHN_UNDEF. In practice, this breaks booting
12.3 kernels on newer loaders, as they do have a kernphys symbol of
the wrong type (NOTYPE, rather than OBJECT) -- we end up deriving the
wrong value for copy_staging.

It's unclear if this version makes any more sense, but it seems to match
what rtld's matched_symbol() does. Loader doesn't need to care about
STT_FUNC w/ UND shndx, because we won't encounter those; in kmods,
undefined (kernel) functions are NOTYPE.

Reported by: Christian McDonald <cmcdonald netgate com>
Reviewed by: imp, kib, tsoome
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D36975