xserver_xorg-server: disable libunwind on MIPS with 32-bit pointers

libunwind support in xorg-server is not only broken for MIPS64 n32 but
for all MIPS variants with 32-bit pointers. The reason is exactly the
same one. libunwind defines a uint64 variable to store the words. Then,
xorg-server casts that variable (64-bit) into a pointer (32-bit), and
the compilation fails:

backtrace.c:98:13: error: cast to pointer from integer of different size
[-Werror=int-to-pointer-cast]

Related:
  https://bugs.freedesktop.org/show_bug.cgi?id=79939

Fixes:
  http://autobuild.buildroot.net/results/87b/87be2c95957f9925c1258812e536df72689fc5da/



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>