xserver_xorg-server: use -O2 build optimization level

xserver_xorg-server: use -O2 build optimization level

The xorg server is somewhat sensitive to certain gcc versions combined
with -Os, particularly for ARM and MIPS, typically in the form of the
following symptom/error:

(EE) Segmentation fault at address 0x0
(EE)
Fatal server error:
(EE) Caught signal 11 (Segmentation fault). Server aborting

Force -O2 to work around this problem - for a target rootfs that uses
X11 the tradeoff is minimal since a sample/test image for qemu vexpress
with X11 + xinit + xterm + twm + xclock + gtk2 with demos is 60+ MB.

And it's less complicated than trying to find all of the bad
combinations.

Fixes bug #8436.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
[Thomas: add comment in .mk file, add reference to bug.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>