Support Big Endian in ICU: part 3

Support Big Endian in ICU:  part 3

Add big endian (mips and mips64) support to BUILD.gn

- icudt[lb]_dat.S (assembly source) is generated at build-time from
  icudt[lb].dat (data bundle) when icu_use_data_file is false.
- icudt[lb].dat is copied depending on the endinaness when icu_use_data_file
  is true.

Part 1:  https://codereview.chromium.org/2162393003
Part 2:  https://codereview.chromium.org/2165403003

It works on Linux (both Chrome and v8) and Mac (v8). Android was also tested
by building base_unittests target with icu_use_data_file=true/false.

With Chrome on Mac, GN port fails even without this CL when
icu_use_data_file is false. (see http://crbug.com/630929). That is likely
to be fixed on the Chromium-side.

Windows should not be affected at all.

BUG=v8:4828
TEST='gn args <builddir>' with icu_use_data_file set to true or false
TEST=build base_unittests and run with --gtest_filter=ICU*
TEST=build base_unittests and run with --gtest_filter=*essage*ormat*
TEST=build 'd8' (v8) and try `(new Date()).toLocaleString("de")`

patch from issue 2174993002 at patchset 20001 (http://crrev.com/2174993002#ps20001)

[Michael Achenbach, 2016-07-26 09:26:56]

Toy CL for testing the v8 trybots.