You can change the behavior of ld with the environment
variable GNUTARGET.
GNUTARGET 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 section BFD). If there is no
GNUTARGET in the environment, ld uses the natural format
of the target. If GNUTARGET 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.