Makefile: use -O3 where possible
Signed-off-by: Paul Beeler <sparksco@gmail.com>

HOSTCFLAGS && HOSTCXXFLAGS use -O3 instead of -O2
Signed-off-by: Paul Beeler <sparksco@gmail.com>

use -O3 for everything:

-now that XZ kernel compression is enabled no need to worry about the size of the kernel.
Signed-off-by: Paul Beeler <sparksco@gmail.com>

use -O3 for MODFLAGS
Signed-off-by: Paul Beeler <sparksco@gmail.com>
defconfig: use XZ kernel compression
Signed-off-by: Paul Beeler <sparksco@gmail.com>