Tell the world we gave up on pushing CC_OPTIMIZE_FOR_SIZE

In commit 281dc5c5ec0f ("Give up on pushing CC_OPTIMIZE_FOR_SIZE") we
already changed the actual default value, but the help-text still
suggested 'y'. Fix the help text too, for all the same reasons.

Sadly, -Os keeps on generating some very suboptimal code for certain
cases, to the point where any I$ miss upside is swamped by the downside.
The main ones are:

- using "rep movsb" for memcpy, even on CPU's where that is
horrendously bad for performance.

- not honoring branch prediction information, so any I$ footprint you
win from smaller code, you lose from less code density in the I$.

- using divide instructions when that is very expensive.

Signed-off-by: Kirill Smelkov <kirr@mns.spb.ru>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>