Silence warning in LTO mode on VxWorks

The link phase is always partial (-r) for VxWorks in kernel mode, which
means that it uses incremental LTO linking by default (-flinker-output=rel).
But in this mode the LTO plugin outputs a warning if one of the object files
involved in the link does not contain LTO bytecode, before switching to
nolto-rel mode.  We do not do repeated incremental linking for VxWorks so
silence the warning.

lto-plugin/
	* lto-plugin.c: Document -linker-output-auto-notlo-rel option.
	(linker_output_set): Change type to bool.
	(linker_output_known): Likewise.
	(linker_output_auto_nolto_rel): New variable.
	(all_symbols_read_handler): Take it into account.
	<LDPO_REL>: Do not issue the warning if it is set.
	(process_option): Process -linker-output-auto-notlo-rel.
	(cleanup_handler): Remove unused variable.
	(onload) <LDPT_LINKER_OUTPUT>: Adjust to above type change.
gcc/
	* gcc.c (LTO_PLUGIN_SPEC): Define if not already.
	(LINK_PLUGIN_SPEC): Execute LTO_PLUGIN_SPEC.
	* config/vxworks.h (LTO_PLUGIN_SPEC): Define.