Add a start/end file specification if the -qrtems option is present.
Allow targets to customize it.

Support the standard -nodefaultlibs option.

gcc/

	* config/rtems.h (RTEMS_STARTFILE_SPEC): Define if undefined.
	(RTEMS_ENDFILE_SPEC): Likewise.
	(STARTFILE_SPEC): Update comment.  Add RTEMS_STARTFILE_SPEC.
	(ENDFILE_SPEC): Add RTEMS_ENDFILE_SPEC.
	(LIB_SPECS): Support -nodefaultlibs option.
	* config/or1k/rtems.h (RTEMS_STARTFILE_SPEC): Define.
	(RTEMS_ENDFILE_SPEC): Likewise.
	* config/rs6000/rtems.h (RTEMS_STARTFILE_SPEC): Likewise.
	(RTEMS_ENDFILE_SPEC): Likewise.
	* config/v850/rtems.h (RTEMS_STARTFILE_SPEC): Likewise.
	(RTEMS_ENDFILE_SPEC): Likewise.