Update GN build files w/ cpu_arch -> current_cpu changes.

R=jam@chromium.org
BUG=344767

Review URL: https://codereview.chromium.org/945723004
diff --git a/BUILD.gn b/BUILD.gn
index 93a6c06..8409da3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -26,10 +26,10 @@
   }
 
   if (is_linux) {
-    if (cpu_arch == "x64") {
+    if (current_cpu == "x64") {
       defines += [ "_FX_CPU_=_FX_X64_" ]
       cflags += [ "-fPIC" ]
-    } else if (cpu_arch == "x86") {
+    } else if (current_cpu == "x86") {
       defines += [ "_FX_CPU_=_FX_X86_" ]
     }
   }