You can change the behavior of the linker with the environment variable GNUPREFIX.
GNUPREFIX determines the input-file object format if you don't use -b (or its synonym --format). Its value should be one of the BFD names for an input format (see Appendix E.). If there is no GNUPREFIX in the environment, the linker uses the natural format of the target. If GNUPREFIX is set to default then BFD attempts to discover the input format by examining binary input files; this method often succeeds, but there are potential ambiguities, since there is no method of ensuring that the magic number used to specify object-file formats is unique. However, the configuration procedure for BFD on each system places the conventional format for that system first in the search-list, so ambiguities are resolved in favor of convention.