Disable some KBUILD CFLAGS:
When -Os was removed in this change 67233197b821534a97310dd050142d7f812aa573
Some flags that were previously disabled by default are no longer disabled.
This causes stability problems.  Instead of using -O2 directly, add in the flags
manually that -O2 enables but not the ones it disables.  Flags that -O2 enables
and -Os disables were taken from GNU documentation:
http://gcc.gnu.org/onlinedocs/gcc/Optimize-Options.html

Signed-off-by: Paul Beeler <sparksco@gmail.com>