This is to fix a linker error when linking icuuc.dll;  ICU_DATA_ENTRY (icudt54_dat) symbol is not found on Windows clean build from the scratch (component=shared_library).

Move stubdata.c to icuuc target from icudata target. Also, make
U_DATA_API (used for U_ICU_DATA_ENTRYPOINT in common/udata.cpp) to be
U_EXPORT instead of U_IMPORT when icu_use_data_file_flag = 1 or
on Windows. On Windows, using the icudt.dll  (i.e. icu_use_data_file_flag=0) also requires this change.

BUG=428145
TEST=All trybots can build a target that requires ICU.
R=mark@chromium.org, scottmg@chromium.org

Review URL: https://codereview.chromium.org/926113004