PowerPC: Fix gettimeofday ifunc selection

PowerPC: Fix gettimeofday ifunc selection

The IFUNC selector for gettimeofday runs before _libc_vdso_platform_setup where
__vdso_gettimeofday is set. The selector then sets __gettimeofday (the internal
version used within GLIBC) to use the system call version instead of the vDSO one.
This patch changes the check if vDSO is available to get its value directly
instead of rely on __vdso_gettimeofday.

This patch changes it by getting the vDSO value directly.

It fixes BZ#16431.