Re-enable LLVM LTO for ARM.

build/toolchain.gypi

 # Copyright 2013 the V8 project authors. 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
 #       copyright notice, this list of conditions and the following
 #       disclaimer in the documentation and/or other materials provided
@@ skipping 259 matching lines @@
                       'USE_EABI_HARDFLOAT=1',
                     ],
                   }],
                   [ 'arm_float_abi=="softfp" or arm_float_abi=="default"', {
                     'defines': [
                       'USE_EABI_HARDFLOAT=0',
                     ],
                   }],
                 ],
               }],
-              # Disable LTO for v8
-              # v8 is optimized for speed, which takes precedence over
-              # size optimization in LTO.
-              ['use_lto==1', {
+              # Disable GCC LTO for v8
+              # v8 is optimized for speed. Because GCC LTO merges flags at link
+              # time, we disable LTO to prevent any -O2 flags from taking
+              # precedence over v8's -Os flag. However, LLVM LTO does not work
+              # this way so we keep LTO enabled under LLVM.
+              ['clang==0 and use_lto==1', {
                 'cflags!': [
                   '-flto',
                   '-ffat-lto-objects',
                 ],
               }],
             ],
           }],  # _toolset=="target"
         ],
       }],  # v8_target_arch=="arm"
       ['v8_target_arch=="arm64"', {
@@ skipping 960 matching lines @@
           ['v8_enable_slow_dchecks==1', {
             'defines': [
               'ENABLE_SLOW_DCHECKS',
             ],
           }],
         ],  # conditions
       },  # Release
     },  # configurations
   },  # target_defaults
 }