Add support for GNAT Ravenscar run-time library.
    
    gdb/ChangeLog (Jerome Guitton, Joel Brobecker):
    
    	* ravenscar-thread.c, ravenscar-thread.h, ravenscar-sparc-thread.c:
    	New files.
    	* configure.tgt (sparc-*-*): Add ravenscar-thread.o and
    	ravenscar-sparc-thread.o to gdb_target_obs.