MIPS SDE OS ABI support

MIPS SDE OS ABI support

This patch is to add SDE OS ABI support in GDB, which has been used in
codesourcery gdb tree for some years.

gdb:

2014-12-19  Maciej W. Rozycki  <macro@codesourcery.com>
	    Nigel Stephens  <nigel@mips.com>
	    Chris Dearman  <chris@mips.com>
	    Luis Machado  <lgustavo@codesourcery.com>

	* Makefile.in (ALL_TARGET_OBS): Add mips-sde-tdep.o.
	(ALLDEPFILES): Add mips-sde-tdep.c.
	* mips-sde-tdep.c: New file containg SDE specific code.
	* configure.tgt (mips*-sde*-elf*): Add mips-sde-dep.o to
	gdb_target_obs.
	* defs.h (gdb_osabi): Add GDB_OSABI_SDE.
	* osabi.c (gdb_osabi_names): Add SDE.
	* NEWS: Mention the change.