selftests/timens: Fix gettime_perf to work on powerpc

On powerpc:
- VDSO library is named linux-vdso32.so.1 or linux-vdso64.so.1
- clock_gettime is named __kernel_clock_gettime()

Ensure gettime_perf tries these names before giving up.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/469f37ab91984309eb68c0fb47e8438cdf5b6463.1617198956.git.christophe.leroy@csgroup.eu