build: Use -O0 for debug build instead of -Og

One of the reasons for using the debug build is to be able to debug
symbols which can't be read in the release build because they are
optimized out:

 (gdb) p var
   $1 = <optimized out>

Unfortunatelly -Og does not prevent this, only -O0 does.

Signed-off-by: Tomasz Grabiec <tgrabiec@cloudius-systems.com>
Signed-off-by: Avi Kivity <avi@cloudius-systems.com>