This patch assigns explicit integers to enum values corresponding

This patch assigns explicit integers to enum values corresponding
to the plugin API.  This patch helps preventing problems while
cherry-picking plugin-api.h changes from trunk gold to local branches.
For instance, a linker plugin compiled with a cherry-picked
plugin-api.h header with some enum values missing will behave
strangely when used with a linker built with the up to date header.

	* plugin-api.h (enum ld_plugin_level): Assign integers
	explicitly for all values.