Rename -Wall to Wsparse-all, so it doesn't get turned on unintentionally

Rename -Wall to Wsparse-all, so it doesn't get turned on unintentionally

sparse's -Wall option turns on all sparse warnings, including those that
many projects will not want; for instance, warnings that enforce
particular stylistic choices, or behavior allowed by a standard but
considered questionable or error-prone.  Furthermore, using -Wall means
accepting all future warnings sparse may start issuing, not just those
intentionally turned on by default.

Other compilers like GCC also use -Wall, and interpret it to mean "turn
on a sensible set of warnings".  Since sparse exists to emit warnings,
it already defaults to emitting a sensible set of warnings.  Many
projects pass the same options to both sparse and the C compiler,
including warning options like -Wall; this results in turning on
excessive amounts of sparse warnings.

cgcc already filtered out -Wall, but many projects invoke sparse
directly rather than using cgcc.  Remove that filter, now that -Wall
does not change sparse's behavior.

Projects almost certainly don't want to use the new -Wsparse-all option;
they should choose the specific set of warnings they want, or just go
with sparse's defaults.

Also update cgcc to know about Wsparse-all and not pass it to GCC, and
update a test case that unnecessarily used -Wall.

Signed-off-by: Josh Triplett <josh@joshtriplett.org>
Signed-off-by: Christopher Li <sparse@chrisli.org>