Add the cc_fuzz target.

Adds the cc_fuzz target via a cc_fuzz module. Also implements the
libclang runtime interface for the x86 toolchain to allow host-built
fuzzers.

Bug: 133261679
Bug: 137398545

Test: Build a fuzzer (with all topic patches), notice that you now
have a host-built fuzzer :)

Change-Id: I7fa069603415f40b3f12a002c253fca6e2aa1988