Add plugin API for processing plugin-added input files.

Add plugin API for processing plugin-added input files.

Gold plugins may wish to further process an input file added by a plugin. For
example, the plugin may need to assign a unique segment for sections in a
plugin-generated input file. This patch adds a plugin callback that the linker
will call when reading symbols from a new input file added after the
all_symbols_read event (i.e. an input file added by a plugin).

2017-11-10  Stephen Crane <sjc@immunant.com>

	* plugin-api.h: Add plugin API for processing plugin-added
	input files.

From-SVN: r254640