Port over binutils fix for PR ld/3111.

This greatly speeds up up linking of object files that contain
lots of dwarf2 symbols by caching symbol tables.

Original patch before binutils was switched to GPLv3 from:
https://sourceware.org/ml/binutils/2006-08/msg00334.html

Aaron Miller made us aware of that diff and adapted it to our
in-tree binutils. Many thanks!

Ports build testing on amd64 by krw@
ok deraadt@ krw@

A first attempt at merging our local changes and fixes into the 2.17 codebase.
ld(1) needs a lot more work, as the upstream code evolved significantly since
2.15, mostly due to changes in PIE binaries layout, and we had already
diverged significantly to implement W^X.

branches: 1.1.1;
Initial revision