libtool: Add support for SerenityOS

libtool: Add support for SerenityOS

This hobbyist OS has already been added to `config.sub` (and
`config.guess` respectively) some time ago, but was still lacking
upstream support for building libraries using libtool.

Since it is a relatively up-to-date system with ports of modern
software, "adding support" mostly just means adding empty cases to avoid
falling though to the most basic behavior (that guarantees compatibility
at the expense of disabling everything that might be critical).

* m4/libtool.m4: Add support for SerenityOS.