assimp: fix m68k uclinux compile

The GCC manual suggest when getting:
relocation truncated to fit: R_68K_GOT16O foobar
to use -mxgot.

https://gcc.gnu.org/onlinedocs/gcc/M680x0-Options.html

Fixes:
http://autobuild.buildroot.net/results/63898c00929b4546279bda52148b218f847714ed/
http://autobuild.buildroot.net/results/bb7a107d1ca6e8713e6ccffe6c61c43b777fb962/



Signed-off-by: Waldemar Brodkorb <wbx@openadk.org>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>