Add a script that generates an assembly file from a .dat file. This is
needed for generating big endian assembly file after using icupkg to
convert little endian icudtl.dat to big endian icudtb.dat. Also the
icu.gyp file is modified so big endian architectures use appropriate
files.

Patch by miran.karic@  ( https://codereview.chromium.org/1967523002/)
with a couple of fixes:

1. Two errors mentioned against PS#9 in the above CL.
2. Support copying icu data file for Big Endian targets.

Besides, icudtb.dat was added to common. icudtb.dat was created by
running 'icupkg -tb icudt56l.dat icudt56b.dat' and renaming icudt56b.dat
to icudtb.dat.

BUG=v8:4828
TEST='d8' is built correct with icu_use_data_file set to either 0 or 1.
TEST=run `GYP_DEFINES="target_arch=mips" ./gypfiles/gyp_v8` and make sure
that ninja files use 'b' data/assembly file for Big Endian.

Review URL: https://codereview.chromium.org/2162393003 .

Patch from Miran Karic <miran.karic@imgtec.com>.