libtool: prevent lto from stripping the magic cookie from the cwrapper

Whole program optimization may remove unused symbols unless told they
are really needed. Fixes sr #108559 reported by LRN.

* build-aux/ltmain.in (func_emit_cwrapperexe_src:MAGIC_EXE): Try to ensure
that the magic cookie is preserved.

Signed-off-by: Peter Rosin <peda@lysator.liu.se>