package/assimp: workaround m68k build issues

package/assimp: workaround m68k build issues

On m68k coldfire, we already pass -mxgot, but since the bump to assimp
5.0.1, this is no longer sufficient, and we have failures such as:

/tmp/ccqmJLil.s: Assembler messages:
/tmp/ccqmJLil.s:307948: Error: value -43420 out of range
/tmp/ccqmJLil.s:307985: Error: value -38606 out of range
/tmp/ccqmJLil.s:308010: Error: value -38626 out of range
/tmp/ccqmJLil.s:308056: Error: value -33280 out of range

Since these issues only arise when building with -O2, let's disable
the optimization for this package on m68k. The very relative relevance
of assimp on m68k coldfire makes the research of a better solution not
really useful (for the record, assimp is a "library to import various
well-known 3D model formats in a uniform manner").

Fixes:

  http://autobuild.buildroot.net/results/a7d4fb2653b0f1be4d036ee46a44e72da0ed4376/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>