LTO plugin: modernize a bit.

include/ChangeLog:

	* plugin-api.h (enum ld_plugin_tag): Do not set implicit enum
	values.

lto-plugin/ChangeLog:

	* lto-plugin.c (struct plugin_objfile): Use bool for offset
	field.
	(exec_lto_wrapper): Assign true/false to bool variables.
	(process_offload_section): Likewise.
	(claim_file_handler): Likewise.
	(onload): Likewise.