i#1569 AArch64: Reimplement ELF import/export iterators.

The previous implementation assumed that all imported symbols are
excluded from the GNU hash table. This assumption seems to be unsafe.
The new implementation iterates through both parts of the symbol table
in both cases, filtering imports or exports using st_value.

Enable client.modules on ARM and AArch64 as it now passes.

Review-URL: https://codereview.appspot.com/306530043

Author: egrimley-arm