Remove legacy 'cc-disable-warning' use from the generic build scripts

This just converts the 'cc-disable-warning' uses in the generic makefile
extrawarn script to use the plain 'cc-option' form instead.  Partly to
clean things up, and partly just to have the new form get some proper
test coverage.

I'll leave the various random other uses of 'cc-disable-warning' alone
and let subsystem maintainers decide if they want to convert to the new
unified model.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>