powerpc/vdso32: Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE

This is copied and adapted from commit 5c929885 ("powerpc/vdso64:
Add support for CLOCK_{REALTIME/MONOTONIC}_COARSE")
from Santosh Sivaraj <santosh@fossix.org>

Benchmark from vdsotest-all:
clock-gettime-realtime: syscall: 3601 nsec/call
clock-gettime-realtime:    libc: 1072 nsec/call
clock-gettime-realtime:    vdso: 931 nsec/call
clock-gettime-monotonic: syscall: 4034 nsec/call
clock-gettime-monotonic:    libc: 1213 nsec/call
clock-gettime-monotonic:    vdso: 1076 nsec/call
clock-gettime-realtime-coarse: syscall: 2722 nsec/call
clock-gettime-realtime-coarse:    libc: 805 nsec/call
clock-gettime-realtime-coarse:    vdso: 668 nsec/call
clock-gettime-monotonic-coarse: syscall: 2949 nsec/call
clock-gettime-monotonic-coarse:    libc: 882 nsec/call
clock-gettime-monotonic-coarse:    vdso: 745 nsec/call

Additional test passed with:
	vdsotest -d 30 clock-gettime-monotonic-coarse verify

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://github.com/linuxppc/issues/issues/41
Link: https://lore.kernel.org/r/d1d24a376e396540194eeb85a2efe481e92ade24.1575273217.git.christophe.leroy@c-s.fr