build: Add cross compile support for wasm*-*-emscripten target.

* configure.ac: Add wasm*-*-emscripten target to generate
lock-obj-pub.native.h.

• Reported-by: Clément Péron <peron.clem@gmail.com>
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>