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.

v8 does not yet support mips/mips64 in GN so that this CL cannot be
tested with target_cpu={mips,mips64}.

With Chrome on Mac, TEST below has to be done with
https://codereview.chromium.org/2181043003 for http://crbug.com/630929.

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=Message*ormat*
TEST=build 'd8' (v8) and try `(new Date()).toLocaleString("de")`
R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/2174993002 .