So right now liblto_plugin.so exports many libiberty symbols and
simple_object file symbols but really it just needs to export onload.

This fixes the problem by using "-export-symbols-regex onload" on
the libtool link line.

lto-plugin/ChangeLog:

	PR lto/49664
	* Makefile.am: Export only onload.
	* Makefile.in: Regenerate.