linux-user, configure: fix (again) syscall_nr.h dependencies cleanup

linux-user, configure: fix (again) syscall_nr.h dependencies cleanup

This patch fixes two problems:
- it cleanups linux-user variants (for instance ppc64-linux-user
  and ppc64le-linux-user)
- it removes the .o file when it removes the .d file, otherwise the .o
  file is never updated

Fixes: 5f29856b852d ("linux-user, configure: improve syscall_nr.h dependencies checking")
Fixes: 4d6a835dea47 ("linux-user: introduce parameters to generate syscall_nr.h")
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200325075757.1959961-1-laurent@vivier.eu>