cygtar: Create Windows hard links (or copies) instead of Cygwin symlinks
cygtar: Create Windows hard links (or copies) instead of Cygwin symlinks

Only Cygwin-built tools understand the Cygwin style of symlinks.
Newer toolchains are built with MinGW instead of Cygwin, so they
do not understand these symlinks.  Proper Windows symlinks cannot
be created without Administrator privilege, so just fall back to
hard links instead.

BUG= https://code.google.com/p/nativeclient/issues/detail?id=4225
TEST= hand tested extracting a toolchain on Windows
R=bradnelson@google.com, binji@chromium.org, sbc@chromium.org

Review URL: https://codereview.chromium.org/1227913002 .

sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client,sbc100/native_client