Run Coverage

We generate the code needed for coverage statistics by adding -Cinstrument-coverage during compilation, and generate coverage data through the integration of minicov and semihosting. Finally, we use the grcov tool to generate readable coverage data in HTML format. All of these have been integrated into our build system, and coverage data can be generated using the following commands：

gn gen out/qemu_riscv64.cov/ --args='build_type="coverage" board="qemu_riscv64"'
ninja -C out/qemu_riscv64.cov check_coverage

If you get a prompt that grcov is not found, you can install it via

cargo install grcov

After building and running, you can find merged coverage report in the ./out/qemu_riscv64.cov/cov_report directory. Open the index.html file in the directory to view the coverage data.