dmalloc: disable on Microblaze with debugging symbols

There is a Microblaze compiler issue when debugging symbols are
enabled, causing assembler errors "Error: operation combines symbols
in different segments". This commit prevents this situation from
happening.

Fixes:

  http://autobuild.buildroot.org/results/d97/d9727e453d7c7c982ce32db5efd455496966e211/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>