RTEMS: Improve GCC specification
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.