configure.ac: Add mips*-*-rtems* support.

2011-02-07  Joel Sherrill <joel.sherrill@oarcorp.com>

	* configure.ac: Add mips*-*-rtems* support.
	* configure: Regenerate.
	* src/mips/ffitarget.h: Ensure needed constants are available
	for targets which do not have sgidefs.h.

From-SVN: r169936