elfcopy: select mode by the end of the program name

The mode of operation (elfcopy, mcs, or strip) is chosen based on the
program name. Broaden this to allow a substring match at the end of the
name to allow prefixes - for example, bsdstrip or aarch64-freebsd-strip.

This improves use of these tools as drop-in replacements for GNU objcopy
and strip, which are often built with a limited set of supported targets
and installed with a target prefix for cross tools.

Reviewed by: dim
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D1663