gofrontend: deduplicate C syscall function declarations

A gccgo language extension allows a function to be declared multiple
times. Avoid the use of this extension by dedeplicating declarations
in mksyscall.awk.

From-SVN: r209508

libgo/go/syscall/mksyscall.awk

@@ -96,8 +96,11 @@ BEGIN {
     cfnresult = line
 
     printf("// Automatically generated wrapper for %s/%s\n", gofnname, cfnname)
-    printf("//extern %s\n", cfnname)
-    printf("func c_%s(%s) %s\n", cfnname, cfnparams, cfnresult)
+    if (!(cfnname in cfns)) {
+        cfns[cfnname] = 1
+        printf("//extern %s\n", cfnname)
+        printf("func c_%s(%s) %s\n", cfnname, cfnparams, cfnresult)
+    }
     printf("func %s(%s) %s%s%s%s{\n",
 	   gofnname, gofnparams, gofnresults == "" ? "" : "(", gofnresults,
 	   gofnresults == "" ? "" : ")", gofnresults == "" ? "" : " ")