ruby: should not use dl_iterate_phdr() on Blackfin

ruby: should not use dl_iterate_phdr() on Blackfin

The dl_iterate_phdr() C library function on Blackfin returns a
non-conventional structure format compared to other architectures,
which makes Ruby unable to use it. Since it's just an optional feature
of Ruby apparently used to generate more fancy backtraces, this commit
simply disables the usage of dl_iterate_phdr() on this architecture.

Fixes:

  http://autobuild.buildroot.org/results/1b0/1b0e2f5f95889eb26846927da45d128247d0ed89/

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