bp2build: uniquify generated header deps in cc_object macro.

Use native.existing_rule to avoid duplicating cc_library_headers from
being generated. While the functionality is known to be brittle and
causes BUILD files to be order dependent, this is sufficient for our
current use case to avoid blowing up the number of targets in the
//bionic/libc package.

Test: bp2build, sync, bazel build //bionic/...
Change-Id: I498afa47780669daffab09a4885a31a80a8fd2b0