RTEMS: Fix GCC specification

RTEMS: Fix GCC specification

The use of -nostdlib and -nodefaultlibs disables the processing of
LIB_SPEC (%L) as specified by LINK_COMMAND_SPEC and thus disables the
default linker script for RTEMS.  Move the linker script to
STARTFILE_SPEC which is controlled by -nostdlib and -nostartfiles.  This
fits better since the linker script defines the platform start file
provided by the board support package in RTEMS.

gcc/

	* config/rtems.h (STARTFILE_SPEC): Remove nostdlib and
	nostartfiles handling since this is already done by
	LINK_COMMAND_SPEC.  Evaluate qnolinkcmds.
	(ENDFILE_SPEC): Remove nostdlib and nostartfiles handling since this
	is already done by LINK_COMMAND_SPEC.
	(LIB_SPECS): Remove nostdlib and nodefaultlibs handling since
	this is already done by LINK_COMMAND_SPEC.  Remove qnolinkcmds
	evaluation.

(cherry picked from commit 28229916e1b131ad5d17a73603f65229e0a92f58)