lint: warn about extern declarations outside headers

https://mail-index.netbsd.org/tech-userlevel/2023/03/15/msg013727.html

lint: revert resolving grammar conflicts for labeled statements

Restore the grammar rule for labeled_statement as it was before cgram.y
1.400 from 2022-04-24. This allows labels with attributes again. Fix
the wrong interpretation in the tests; the attributes belong to the
label, not to the statement.

Today in the morning, when I thought that the change in cgram.y 1.400
were innocent, I accidentally ran lint only with the options '-Sw' but
forgot the option '-g' for GNU mode. Without that option, the token
'__attribute__' is unknown, which unsurprisingly leads to lots of syntax
errors, and these didn't change with that commit. The actual change was
only visible in GNU mode.

tests/lint: add test for GCC unused statement

Seen in xsrc/LRGB.c:799.

Even though it looks like this lint error may have been introduced by
cgram.y 1.400 from 2022-04-24 (since LRGB.c has been unchanged for a
month), earlier versions of lint produce the same parse errors.

tests/lint: add tests for GCC __attribute__

Before fixing the wrong handling of __attribute__ that is demonstrated
at the end of gcc_attribute.c, ensure that the attribute handling works
in the most basic cases.

Lint currently accepts __attribute__ in more places than it should.
This leads to some ambiguities in the grammar.