Rename a64 target arch to arm64 in blink_heap.gypi

Source/heap/blink_heap.gypi

 #
 # Copyright (C) 2013 Google Inc. All rights reserved.
 #
 # Redistribution and use in source and binary forms, with or without
 # modification, are permitted provided that the following conditions are
 # met:
 #
 #     * Redistributions of source code must retain the above copyright
 # notice, this list of conditions and the following disclaimer.
 #     * Redistributions in binary form must reproduce the above
@@ skipping 32 matching lines @@
     ],
     'heap_test_files': [
       'HeapTest.cpp',
     ],
     'conditions': [
       ['target_arch == "arm"', {
         'heap_asm_files': [
           'asm/SaveRegisters_arm.S',
         ],
       }],
-      ['target_arch == "a64"', {
+      ['target_arch == "arm64"', {
         'heap_asm_files': [
-          'asm/SaveRegisters_a64.S',
+          'asm/SaveRegisters_arm64.S',
         ],
       }],
       ['target_arch == "mipsel"', {
         'heap_asm_files': [
           'asm/SaveRegisters_mips.S',
         ],
       }],
       ['target_arch == "ia32" or target_arch == "x64"', {
         'heap_asm_files': [
           'asm/SaveRegisters_x86.asm',
         ],
       }],
     ],
   },
 }