[third_party][boringssl] Limit unsafe lib visibiliaty

This CL restricts the fuzzed versions of libcrypto and libssl to only
be available to the fuzzers. It also tries to more gracefully handle
the name collision between the fuzzers_package template and the
generated .gni file.

Change-Id: I849c2406c1e574f34c591cbfc41a12a08ce02d8a