extrawarn: enable format and stringop overflow warnings in W=1

The stringop and format warnings got disabled globally when they were
newly introduced in commit bd664f6b ("disable new gcc-7.1.1 warnings
for now"), 217c3e01 ("disable stringop truncation warnings for now")
and 5a76021c ("gcc-10: disable 'stringop-overflow' warning for now").

In all cases, the sentiment at the time was that the warnings are
useful, and we actually addressed a number of real bugs based on
them, but we never managed to eliminate them all because even the
build bots using W=1 builds only see the -Wstringop-truncation
warnings that are enabled at that level.

Move these into the W=1 section to give them a larger build coverage
and actually eliminate them over time.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>