Fix 32-bit x86 Linux build for GN uploads.

The bots still use 'cpu_arch' to set the architecture, and the build
file was keying off of build_cpu_arch to determine whether or not
to enable NaCl, so things were failing. Updating the the code to
'current_cpu' fixes the problem correctly.

TBR=brettw@chromium.org

Review URL: https://codereview.chromium.org/964003002

Cr-Commit-Position: refs/heads/master@{#318585}

build/config/features.gni

@@ -32,8 +32,8 @@ declare_args() {
   # the commented out logic.
   # Eventually we want this to be:
   #   enable_nacl = !is_ios && !is_android
-  enable_nacl = (is_linux && !is_chromeos && !is_debug &&
-                 build_cpu_arch == "x64") || is_nacl
+  enable_nacl =
+      (is_linux && !is_chromeos && !is_debug && current_cpu == "x64") || is_nacl
   enable_nacl_untrusted = enable_nacl
   enable_pnacl = enable_nacl_untrusted