Limiting SwiftShader compilation to x64 and x86 architectures

BUG=630728

Review-Url: https://codereview.chromium.org/2568283002
Cr-Commit-Position: refs/heads/master@{#438145}

BUILD.gn

@@ -527,7 +527,8 @@ group("both_gn_and_gyp") {
     ]
   }
 
-  if (is_win || is_mac || is_linux) {
+  if ((is_win || is_mac || is_linux) &&
+      (target_cpu == "x86" || target_cpu == "x64")) {
     deps += [ "//third_party/swiftshader" ]
   }