Update ARFLAGS?

[Thomas Klausner <wiz%NetBSD.org@localhost>]

Hi!

As noted in PR 57565, the default ARFLAGS in share/mk/sys.mk are
broken - they use 'l' which changed behaviour between binutils 2.34
and 2.39.

Ok to commit the change?

(This broke the build of ruby-nokogiri recently, which is how I
noticed.)
 Thomas