Follow-up CL to https://codereview.chromium.org/2162393003

1. make_data_assembly now accepts '--mac' to generate assembly source
for Mac

2.  Fix icu.gyp to support all platforms

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

R=machenbach@chromium.org

Review URL: https://codereview.chromium.org/2165403003 .