Log Message:
ruby: Fix extension installs on non-NetBSD.

For some reason NetBSD does not append an OS version like pretty much all other
OS, and so installs of arch-specific extensions directories were broken due to
the triple containing a dash before the version and causing PLIST failures.

Introduce a new RUBY_EXTARCH variable that has a special-case for NetBSD, and
use it for GEM_EXTSBASE.  Fixes many packages, but notably ruby31-base.