build: On cross build, detect host's objdump.

* configure.ac [*-*-linux*] (CROSS_HOST_OBJDUMP): New.

To support widely, we could also use objdump configured for multiple
archs which supports host's format (instead of $host_alias-objdump),
like objdump in binutils-multiarch of Debian, but it's not that easy
to maintain the code to determine format name from $host_alias.

Or we could use eu-size in elfutils.

• GnuPG-bug-id: T4973
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>