bnd: Add a new nobnd prefix

bnd and nobnd prifixes can be used for each instruction line to
direct whether bnd registers should be preserved or not.

And those are also added as options for DEFAULT directive.
Once bnd is set with default, DEFAULT BND, all bnd-prefix
available instructions are prefixed with bnd. To override it,
nobnd prefix can be used.

In the other way, DEFAULT NOBND can disable DEFAULT BND and
have nasm encode in the normal way.

Signed-off-by: Jin Kyu Song <jin.kyu.song@intel.com>

parser.c

@@ -90,6 +90,7 @@ static int prefix_slot(int prefix)
     case P_XACQUIRE:
     case P_XRELEASE:
     case P_BND:
+    case P_NOBND:
         return PPS_REP;
     case P_O16:
     case P_O32: