Makefile: Try dynamic linkage for bfd

On Debian Stretch/Ubuntu 14.04, the libbfd provided by libbfd-dev or
binutils-dev packages does not like being linked statically.

Add a dynamic linkage test when detecting libbfd.

Signed-off-by: Julien Thierry <julien.thierry@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>