Rust: Collect gnco files with CLANG_COVERAGE too.

Until Rust supports LLVM coverage, Soong treats both NATIVE_COVERAGE and
CLANG_COVERAGE as producing GCOV coverage files. On the make-side, we
should be collecting the gcno files into a zip file when CLANG_COVERAGE
is defined too.

Bug: 170983066
Test: Coverage build with CLANG_COVERAGE produces zip file.
Change-Id: If6f86280f0d823e86884b64729530cc3548103a1