elfcopy: Use libelftc's string table routines to build .shstrtab.

This replaces some hand-rolled routines and is substantially faster
since libelftc uses a hash table for lookups and insertions, whereas
elfcopy would perform a linear scan of the table.

PR: 234949
Reviewed by: emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D20473

elfcopy: correct comment typo in r304151

elfcopy: silence GCC 5.3 unitialized variable warning

Although it's a false positive there is little cost to initializing it
always.

Submitted by: adrian

Fix elftoolchain tools in-tree build

* make variables static
* add header for uint*_t typedefs