Check for prefixed ar, readelf, and nm

Check for prefixed ar, readelf, and nm

Sometimes with cross-compile toolchains, the tools are prefixed with the
target arch. Using AC_CHECK_TOOL looks for tools named like this.

Signed-off-by: Michael Forney <mforney@mforney.org>
Signed-off-by: Mark Wielaard <mjw@redhat.com>