Use -mthumb on debug build too.

SConstruct

 # Copyright 2008 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 75 matching lines @@
     'CPPDEFINES':   ['ENABLE_LOGGING_AND_PROFILING']
   },
   'gcc': {
     'all': {
       'CCFLAGS':      ['$DIALECTFLAGS', '$WARNINGFLAGS'],
       'CXXFLAGS':     ['$CCFLAGS', '-fno-rtti', '-fno-exceptions'],
     },
     'mode:debug': {
       'CCFLAGS':      ['-g', '-O0'],
       'CPPDEFINES':   ['ENABLE_DISASSEMBLER', 'DEBUG']
+      'os:android': {
+        'CCFLAGS':    ['-mthumb']
+      }
     },
     'mode:release': {
       'CCFLAGS':      ['-O3', '-fomit-frame-pointer', '-fdata-sections',
                        '-ffunction-sections'],
       'os:android': {
         'CCFLAGS':    ['-mthumb', '-Os'],
         'CPPDEFINES': ['SK_RELEASE', 'NDEBUG']
       }
     },
     'os:linux': {
@@ skipping 649 matching lines @@
 # version of scons.  Also, there's a bug in some revisions that
 # doesn't allow this flag to be set, so we swallow any exceptions.
 # Lovely.
 try:
   SetOption('warn', 'no-deprecated')
 except:
   pass
 
 
 Build()