Left over .gcno files from old builds can really confuse gcov and the
user expects a clean slate after "make clean". Make clean mean clean.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220204204335.1689602-4-alex.bennee@linaro.org>