Cygwin/MinGW: Do not version lto plugins

GCC on Linux already uses liblto_plugin.so directly without
the libtool version suffix, adjust windows GCC to do the same.

gcc/ChangeLog:

	* config.host: Adjust plugin name for Windows.

lto-plugin/ChangeLog:

	* Makefile.am: drop versioning from libtool completely.
	* Makefile.in: regenerate.