powerpc/vdso: Merge vdso64 and vdso32 into a single directory

merge vdso64 into vdso32 and rename it vdso.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/4dbe05cc130f6a0858d09ac72e436c373cb08b70.1642782130.git.christophe.leroy@csgroup.eu