Fix standalone VM build for Android for tcmalloc.

tools/gyp/configurations_android.gypi

 # Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 # for details. All rights reserved. Use of this source code is governed by a
 # BSD-style license that can be found in the LICENSE file.
 
 # Definitions for building standalone Dart binaries to run on Android.
 # This is mostly excerpted from:
 # http://src.chromium.org/viewvc/chrome/trunk/src/build/common.gypi
 
 {
   'variables': {
@@ skipping 27 matching lines @@
               '__GNU_SOURCE=1',
               '_STLP_USE_PTR_SPECIALIZATIONS=1',
               'HAVE_OFF64_T',
               'HAVE_SYS_UIO_H',
             ],
             'cflags!': [
               '-pthread',  # Not supported by Android toolchain.
             ],
             'cflags': [
               '-U__linux__',  # Don't allow toolchain to claim -D__linux__
+              '-U__linux',
               '-ffunction-sections',
               '-funwind-tables',
               '-fstack-protector',
               '-fno-short-enums',
               '-finline-limit=64',
               '-Wa,--noexecstack',
             ],
           }],
         ],
       },
@@ skipping 256 matching lines @@
             ],
           }],
           ['_toolset=="host"', {
             'ldflags': [ '-pthread' ],
           }],
         ],
       },  # Dart_Android_arm64_Base
     },  # configurations
   },  # target_defaults
 }