Fix GDB BFD test linking

When compiling GDB for target (in my case i386) it links
wrong BFD library from host OS. This prevents GDB from compiling
support for ELF and thus GDB is unusable on target.

More about this issue was already posted at:
http://lists.uclibc.org/pipermail/buildroot/2009-March/026585.html



Fix this issue by forcing ELF support.

Signed-off-by: Paulius Zaleckas <paulius.zaleckas@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>