[zircon][gn] Add fuzzer environments

This CL adds environments derived from the standard environments that
instrument code with '-fsanitize=fuzzer' and link against
libclang_rt.fuzzer.a.

Bug: SEC-251
Test: noop-test; see zircon/public/gn/toolchain/environment.gni
Change-Id: I7d25134b78eb2fbdead551599e15218b68ecd5ba