Extend getrandom() vDSO implementation to VDSO64.

Tested on QEMU on both ppc64_defconfig and ppc64le_defconfig.

Results from a Power9 (PowerNV):
~ # ./vdso_test_getrandom bench-single
   vdso: 25000000 times in 0.787943615 seconds
   libc: 25000000 times in 14.101887252 seconds
   syscall: 25000000 times in 14.047475082 seconds
Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Tested-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>