rust: Fix incorrect expected gcno file for dylibs

For Rust dylibs we generate a file suffix of .dylib.so. rustc seems to
split the output filename for the gcno file at the first '.', so instead
of libfoo.dylib.gcno (which our build system expects), it produces
libfoo.gcno.

This fixes the mismatch so the build system can find the gcno file that
gets produced for Rust dylibs.

Bug: 170983066
Test: CLANG_COVERAGE=true NATIVE_COVERAGE_PATHS=external/rust/crates \
      m libandroid_log_sys
Change-Id: Icba48bf10ab06c255bd17053d57feb889b1f9ee5