qt5base: fix double-conversion for or1k

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/a3535cdf5e91df011a59a4b9f60d69195f5efdcb



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>