[zircon][gn] Don't use -Og for opt_level==0

Clang's -Og doesn't actually do the thing it's intended for,
and it's broken in combination with -flto.  Just use -O0 for
both opt_level=0 and opt_level=-1.

Bug: TC-472 #comment Disabled use of -Og in Zircon
Change-Id: Ia4cc9d61b1857644d4da495b855550789589950d