RTEMS: Fix -Werror builds

RTEMS: Fix -Werror builds

Fix build errors due to warnings such as:

gcc/config/v850/rtems.h:43: error: "RTEMS_STARTFILE_SPEC" redefined [-Werror]
   43 | #define RTEMS_STARTFILE_SPEC ""

The problem was that "gcc/config/rtems.h" was included before the
architecture-specific "gcc/config/*/rtems.h" header file on some
architectures.

gcc/

	* config.gcc (aarch64-*-rtems*): Include general rtems.h after
	the architecture-specific rtems.h.
	(aarch64-*-rtems*): Likewise.
	(arm*-*-rtems*): Likewise.
	(epiphany-*-rtems*): Likewise.
	(riscv*-*-rtems*): Likewise.