[boringssl] Better encapsulate the Zircon BoringSSL build.

This CL removes some ununsed files and extracts the source lists needed
by the Zircon build into a 'zircon.gni' file. This allows the upstream
code and the source lists to be updated atomically.

Bug: 41904
Change-Id: Ibc61ba48424a620ed62e47e40aa9a6c1e673fa49
4 files changed