MIPS64: Fix MIPS64 related issues in GN files

third_party/WebKit/Source/platform/heap/asm/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
 if (current_cpu == "x86" || current_cpu == "x64") {
   import("//third_party/yasm/yasm_assemble.gni")
 
   yasm_assemble("asm") {
     assert(current_cpu == "x86" || current_cpu == "x64")
 
@@ skipping 24 matching lines @@
         "SaveRegisters_arm.S",
       ]
     } else if (current_cpu == "arm64") {
       sources = [
         "SaveRegisters_arm64.S",
       ]
     } else if (current_cpu == "mipsel") {
       sources = [
         "SaveRegisters_mips.S",
       ]
+    } else if (current_cpu == "mips64el") {
+      sources = [
+        "SaveRegisters_mips64.S",
+      ]
     }
 
     if (current_cpu == "arm") {
       defines = [ "ARM=1" ]
     }
   }
 }  # current_cpu == "x86" || current_cpu == "x64"