Switch gdb's TRY/CATCH to sjlj again

Switch gdb's TRY/CATCH to sjlj again

We don't currently handle the case of gdb's readline callback throwing
gdb C++ exceptions across a readline that wasn't built with
-fexceptions.  The end result is:

 (gdb) whatever-command-that-causes-an-error
 terminate called after throwing an instance of 'gdb_exception_RETURN_MASK_ERROR'
 Aborted
 $

Until that is fixed, revert back to sjlj-based exceptions again.

gdb/ChangeLog:
2016-04-21  Pedro Alves  <palves@redhat.com>

	* common/common-exceptions.h (GDB_XCPT_TRY): Add comment.
	(GDB_XCPT): Always define as GDB_XCPT_SJMP.