elfcopy: exclude extension when converting from binary

When converting from binary to ELF, elfcopy creates symbols
_binary_<filename>_start_, _binary_<filename>_end, and
_binary_<filename>_size. For compatibility with GNU objcopy (and to
produce sensible symbol names) the extension must be stripped off.

Reviewed by: imp
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D4238