Move GN arg delcarations into their own declare_args call

GN throws an error when they are declared and used in the same call.

Bug: None
Change-Id: I2eb5fdff585071a345d2765e91c69cb41180b685
Reviewed-on: https://chromium-review.googlesource.com/665298
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Derek Schuff <dschuff@chromium.org>
diff --git a/config.gni b/config.gni
index 076e7c7..18b47c1 100644
--- a/config.gni
+++ b/config.gni
@@ -7,11 +7,12 @@
   use_nacl_x86 = target_cpu == "x86"
   use_nacl_x64 = target_cpu == "x64" || target_os == "win"
   use_nacl_arm = target_cpu == "arm"
-
   # Untrusted libc
   use_gcc_glibc = false
   use_clang_newlib = true
+}
 
+declare_args() {
   if (use_nacl_x86) {
     if (use_gcc_glibc) {
       toolchain_untrusted = "//build/toolchain/nacl:glibc_x86"