vdso: add support for GNU hash tables

on some kernel builds, the vdso exports symbols with DT_GNU_HASH but
omits DT_HASH, causing the vdso not to get used and for clock_gettime
to fall back to using a syscall.

our vdso resolver does not use the hash table anyway, but does need
the symbol count from the standard sysv hash table. if it's missing,
use the GNU hashtable to calculate the number of symbols.