Fix bug in 64bit -> 32bit cross build

[jiangzidong]

When cross-build 32bit version iotjs in 64bit host, the ./tools/build.py --target-arch=i686 will not succeed, because the "-m32" or "-march=i686" flag is not set in the cmake/config/i686-linux.cmake.

This patch fixs the bug.

1, add "-m32" or "-march=i686" flag in i686-linux.cmake
2, libuv is built by gyp. So we modified the tools/build.py to pass the target_arch to the gyp_uv
3, libtuv use the opt_cmake_toolchain_file() as its CMAKE_TOOLCHAIN_FILE, like other dep_modules did. so libtuv can also support i686 cross-build.

[seanshpark]

@jiangzidong , I have changed libtuv so that to cross compile i686 in x86_64.
You can remove two changes

-                            join_path([LIBTUV_ROOT, 'cmake/config/config_' +
-                                      opt_target_tuple() + ".cmake"]))
+                             opt_cmake_toolchain_file())

+        check_run_cmd('cmake', libtuv_cmake_opt)

[jiangzidong]

Hi, @seanshpark Should I 1) add a new commit after this, or 2) git commit --amend and push force, or 3) open a new PR?

[seanshpark]

@jiangzidong , I think 2) will do.

[jiangzidong]

@seanshpark Thanks, and I have updated

[seanshpark]

@jiangzidong , Thank you. I'll check and merge.

[seanshpark]

@jiangzidong , Can you please rebase to latest master? There was an update for libtuv.

[jiangzidong]

@seanshpark Sorry that I forgot to sync. What about now?

[seanshpark]

@jiangzidong , fine, thank you :)