Add support for Semihosting v2 support for AArch64 in libgloss.

Semihosting v2 changes are documented here:
https://developer.arm.com/docs/100863/latest/

The biggest change is the addition of an extensions mechanism
to add more extensions in the future.

Signed-off-by: Tamar Christina <tamar.christina@arm.com>

libgloss/aarch64/_exit.c

@@ -36,6 +36,12 @@ _exit (int status)
       param_block_t block[2];
       block[0] = ADP_Stopped_ApplicationExit;
       block[1] = status;
-      do_AngelSVC (AngelSVC_Reason_ReportException, block);
+
+#if SEMIHOST_V2
+      if (_has_ext_exit_extended ())
+	 do_AngelSVC (AngelSVC_Reason_ReportExceptionExtended, block);
+      else
+#endif
+	 do_AngelSVC (AngelSVC_Reason_ReportException, block);
     }
 }