build: fix icu support when target arch != host

Without this change, I get the following error when running configure with `--dest-cpu=ia32` on a x64 system:
```
➜  node git:(srl-v0.12-autoicu) ✗ ./configure --dest-cpu=ia32 --with-intl=small-icu
creating  ./icu_config.gypi
* Using ICU in ./deps/icu
creating  ./icu_config.gypi
{ 'target_defaults': { 'cflags': [],
                       'default_configuration': 'Release',
                       'defines': ['OPENSSL_NO_SSL2=1'],
                       'include_dirs': [],
                       'libraries': []},
  'variables': { 'clang': 1,
                 'host_arch': 'x64',
                 'icu_data_file': 'icudt54l.dat',
                 'icu_data_in': '../../deps/icu/source/data/in/icudt54l.dat',
                 'icu_endianness': 'l',
                 'icu_gyp_path': 'tools/icu/icu-generic.gyp',
                 'icu_path': './deps/icu',
                 'icu_small': 'true',
                 'icu_ver_major': '54',
                 'node_install_npm': 'true',
                 'node_prefix': '',
                 'node_shared_cares': 'false',
                 'node_shared_http_parser': 'false',
                 'node_shared_libuv': 'false',
                 'node_shared_openssl': 'false',
                 'node_shared_v8': 'false',
                 'node_shared_zlib': 'false',
                 'node_tag': '',
                 'node_use_dtrace': 'true',
                 'node_use_etw': 'false',
                 'node_use_mdb': 'false',
                 'node_use_openssl': 'true',
                 'node_use_perfctr': 'false',
                 'openssl_no_asm': 0,
                 'python': '/usr/bin/python',
                 'target_arch': 'ia32',
                 'uv_library': 'static_library',
                 'uv_parent_path': '/deps/uv/',
                 'uv_use_dtrace': 'true',
                 'v8_enable_gdbjit': 0,
                 'v8_enable_i18n_support': 1,
                 'v8_no_strict_aliasing': 1,
                 'v8_optimized_debug': 0,
                 'v8_random_seed': 0,
                 'v8_use_snapshot': 'true',
                 'want_separate_host_toolset': 1}}
creating  ./config.gypi
creating  ./config.mk
gyp: Dependency '/Users/JulienGilli/dev/node/tools/icu/icu-generic.gyp:icuuc#target' not found while trying to load target /Users/JulienGilli/dev/node/deps/v8/tools/gyp/v8.gyp:v8_base#target
Error running GYP
➜  node git:(srl-v0.12-autoicu) ✗
```

The change in this PR fixes it, but I'm not sure it's the proper way to do it.