[cc_fuzz] Statically link libcxx.

Statically links libcxx in order to not require shared library
deployment during packing of fuzz targets.

Test: m example_fuzzer && ldd $ANDROID_HOST_OUT/fuzz/example_fuzzer.
Note that the ldd results don't include libc++.so.
Bug: 139880425

Change-Id: I86d1945768de53217d54cb762392c20714494416