gas: Default to ELF for RTEMS targets

	* configure.tgt (aarch64*-*-rtems*): Remove.
	(bfin-*-rtems*): Likewise.
	(h8300-*-rtems*): Likewise.
	(i386-*-rtems*): Likewise.
	(m32c-*-rtems*): Likewise.
	(m32r-*-rtems*): Likewise.
	(m68k-*-rtems*): Likewise.
	(mips-*-rtems*): Likewise.
	(nios2-*-rtems*): Likewise.
	(ppc-*-rtems*): Likewise.
	(sh-*-rtems*): Likewise.
	(sparc64-*-rtems*): Likewise.
	(sparc-*-rtems*): Likewise.
	(*-*-rtems*) Use ELF format.