arm64: clean vdso & vdso32 files

arm64: clean vdso & vdso32 files

commit c62bf1412063 ("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: c62bf1412063 ("arm64: do not descend to vdso directories twice")
Link: https://lore.kernel.org/r/20210810231755.1743524-1-adelg@google.com
Signed-off-by: Will Deacon <will@kernel.org>