I don't see any reason to treat nonstandard section types specially at all.
The norm is to strip non-allocated sections. We have special exceptions
for SHT_NOTE and a few named SHT_PROGBITS sections that particularly should
not be stripped. If there are any nonstandard types that should not be
stripped, then we can add special cases for those.
Unless anyone can cite a good reason not to, I'll just remove the
< SHT_NUM check entirely.