Allow building with CFLAGS="-O0"

If compiled with -O0, gcc doesn't do any stack frame coalescing
and -Wframe-larger-than=1024 is triggered in debug mode.
Starting with gcc 4.8, new opt level -Og is introduced for debugging, which
does not trigger this warning.

Fix bench zio size, using SPA_OLD_MAXBLOCKSHIFT

Signed-off-by: Gvozden Neskovic <neskovic@gmail.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4799