linux-user, configure: improve syscall_nr.h dependencies checking

This is mostly a fix for in-tree build.

It removes errors on .d directories:

  grep: ./.gitlab-ci.d: Is a directory
  grep: ./scripts/qemu-guest-agent/fsfreeze-hook.d: Is a directory

and improves performance by only checking <ARCH>-linux-user directories.

Reported-by: BALATON Zoltan <balaton@eik.bme.hu>
Tested-by: BALATON Zoltan <balaton@eik.bme.hu>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20200323114116.163609-1-laurent@vivier.eu
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>

configure

@@ -1910,9 +1910,9 @@ for arch in alpha hppa m68k xtensa sh4 microblaze arm ppc s390x sparc sparc64 \
     # remove the file if it has been generated in the source directory
     rm -f "${source_path}/linux-user/${arch}/syscall_nr.h"
     # remove the dependency files
-    find . -name "*.d" \
-           -exec grep -q "${source_path}/linux-user/${arch}/syscall_nr.h" {} \; \
-           -exec rm {} \;
+    test -d ${arch}-linux-user && find ${arch}-linux-user -type f -name "*.d" \
+         -exec grep -q "${source_path}/linux-user/${arch}/syscall_nr.h" {} \; \
+         -exec rm {} \;
 done
 
 if test -z "$python"