qt5base: fix double conversion for microblazeel/microblazebe

Fixes [1]:

  ../3rdparty/double-conversion/include/double-conversion/utils.h:81:2: error: #error Target architecture was not detected as supported by Double-Conversion.
  #error Target architecture was not detected as supported by Double-Conversion.

[1] http://autobuild.buildroot.net/results/489/4891d96f45c64c2e66fe819bd4175cc1d6243a93



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>