package/icu: fix double conversion compile for microblaze

package/icu: fix double conversion compile for microblaze

Fixes [1]:

  In file included from double-conversion.h:42:0,
                   from number_decimalquantity.cpp:19:
  double-conversion-utils.h:120:2: error: #error Target architecture was not detected as supported by Double-Conversion.
   #error Target architecture was not detected as supported by Double-Conversion.

Apply the same fix as previously for qt5base ([2], already sent upstream [3],
but seems to be lost while merging [4]).

[1] http://autobuild.buildroot.net/results/24005bc54cbc742decee221fb09bca19ee70502c
[2] https://git.buildroot.net/buildroot/commit/?id=530668ddb709cf27aa6694e00f44cf58af858b0b
[3] https://github.com/google/double-conversion/issues/73
[4] https://github.com/google/double-conversion/commit/768a445f0c28311bf88685bf0bb990505c12fd4c

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