pdbg: add patch fixing the build on Blackfin

This commit adds a patch to pdbg that fixes the build on the Blackfin
architecture. The build failure was due to the recently introduced
assembly code to embed the DTB into an object file. This code was not
taking into account the fact that Blackfin has a non-empty
__USER_LABEL_PREFIX__.

The patch has been submitted upstream.

Fixes:

  http://autobuild.buildroot.net/results/2bf6f56303453fd2ba7e86882168d406ded4cc80/



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