[NDS32] Complete -march= option design so that it can display available items...

[NDS32] Complete -march= option design so that it can display available items in --target-help message.

gcc/
	* config/nds32/nds32.opt (march): Add help message.

From-SVN: r217901

gcc/ChangeLog

+2014-11-21  Chung-Ju Wu  <jasonwucj@gmail.com>
+
+	* config/nds32/nds32.opt (march): Add help message.
+
 2014-11-20  Patrick Palka  <ppalka@gcc.gnu.org>
 	* tree-vrp.c (test_for_singularity): New parameter

gcc/config/nds32/nds32.opt

@@ -71,6 +71,7 @@ Specify the name of the target architecture.
 
 Enum
 Name(nds32_arch_type) Type(enum nds32_arch_type)
+Known arch types (for use with the -march= option):
 
 EnumValue
 Enum(nds32_arch_type) String(v2) Value(ARCH_V2)