arm64: clean vdso & vdso32 files

commit a5b8ca97 ("arm64: do not descend to vdso directories twice")
changes the cleaning behavior of arm64's vdso files, in that vdso.lds,
vdso.so, and vdso.so.dbg are not removed upon a 'make clean/mrproper':

$ make defconfig ARCH=arm64
$ make ARCH=arm64
$ make mrproper ARCH=arm64
$ git clean -nxdf
Would remove arch/arm64/kernel/vdso/vdso.lds
Would remove arch/arm64/kernel/vdso/vdso.so
Would remove arch/arm64/kernel/vdso/vdso.so.dbg

To remedy this, manually descend into arch/arm64/kernel/vdso upon
cleaning.

After this commit:
$ make defconfig ARCH=arm64
$ make ARCH=arm64
$ make mrproper ARCH=arm64
$ git clean -nxdf
<empty>

Similar results are obtained for the vdso32 equivalent.
Signed-off-by: Andrew Delgadillo <adelg@google.com>
Cc: stable@vger.kernel.org
Fixes: a5b8ca97 ("arm64: do not descend to vdso directories twice")
Link: https://lore.kernel.org/r/20210810231755.1743524-1-adelg@google.comSigned-off-by: Will Deacon <will@kernel.org>

arch/arm64/Makefile

@@ -183,6 +183,8 @@ endif
 # We use MRPROPER_FILES and CLEAN_FILES now
 archclean:
 	$(Q)$(MAKE) $(clean)=$(boot)
+	$(Q)$(MAKE) $(clean)=arch/arm64/kernel/vdso
+	$(Q)$(MAKE) $(clean)=arch/arm64/kernel/vdso32
 
 ifeq ($(KBUILD_EXTMOD),)
 # We need to generate vdso-offsets.h before compiling certain files in kernel/.