gas: make as_tsktsk() output more as_warn()-like

The lack of a uniform "Warning: " prefix can be irritating. Re-use
as_warn_internal(), by moving the warning count increment into the pre-
existing callers (where the flag_no_warnings checks also are). At the
same time keep the listing_warning() invocation at its place - listings
certainly should have such warnings reproduced as well.

While there also drop the unnecessary forward declarations of static
functions.