Delete three pre-built assembly source files because they're now
generated at build-time.

Update data build scripts and README.chromium accordingly.

Update copy_data.sh and copy_data_android.sh so that the assembly
source files are not copied. Besides, convert the little endian
data bundle to the big endian data bundle for non-Android platforms.

BUG=v8:4828
TEST=Rebuild icu data following the procedure in README.chromium
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")`

Review URL: https://codereview.chromium.org/2182883004 .