aarch64: Emit +rdm when the platform asm does not support +rdma.

Although the two extension names are aliases, they are not supported
uniformly across assemblers.

This emits +rdm for assemblers that do not support +rdma.

TODO: Wrap the change in a conditional and supply that from config.gcc
on the basis of configuration.

gcc/ChangeLog:

	* common/config/aarch64/aarch64-common.cc: Always emit +rdm
	even when the given extension name is rdma.

Signed-off-by: Iain Sandoe <iain@sandoe.co.uk>

gcc/common/config/aarch64/aarch64-common.cc

@@ -333,7 +333,10 @@ aarch64_get_extension_string_for_isa_flags
     if (added & opt.flag_canonical)
       {
 	outstr += "+";
-	outstr += opt.name;
+	if (startswith (opt.name, "rdm"))
+	  outstr += "rdm";
+	else
+	  outstr += opt.name;
       }
 
   /* Remove the features in current_flags & ~isa_flags.  If the feature does
@@ -362,7 +365,10 @@ aarch64_get_extension_string_for_isa_flags
 	{
 	  current_flags &= ~opt.flags_off;
 	  outstr += "+no";
-	  outstr += opt.name;
+	  if (startswith (opt.name, "rdm"))
+	    outstr += "rdm";
+	  else
+	    outstr += opt.name;
 	}
     }