xserver_xorg-server: libunwind support is broken on MIPS64 n32

Amend the former commit and disable xorg-server with libunwind support
only for MIPS64 n32, as stated in the bug report:

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



Signed-off-by: Vicente Olivert Riera <Vincent.Riera@imgtec.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>