Enable full debug builds by having ar creating thin archives

By default, CMake uses ar to generate libWebCore.a with cr parameters
(do not warn if the library has to be created, and replace existing
files in the archive). That results in a very large file, and ar fails
with sizes over 4GB.

By using thin archives (the T option of ar), the generated
libWebCore.a is much smaller, and can be successfully archived. As a
result, we can generate a full debug build. This patch is based on Xan
López's upstream webkit.org/b/110580.