[llvm-objcopy] Add support for large indexes

This patch is an update of an older patch that never landed
(see here: https://reviews.llvm.org/D42516)

Recently various users have run into this issue and it just 100%
has to be solved at this point. The main difference in this patch
is that I use gunzip instead of unzip which should hopefully allow
tests to pass. Please review this as if it is a new patch however.
I found some issues along the way and made some minor modifications.

The binary used in this patch for testing (a zip file to make it small)
can be found here:
https://drive.google.com/file/d/1UjsnTO9edLttZibbr-2T1bJl92KEQFAO/view?usp=sharing

Differential Revision: https://reviews.llvm.org/D49206

llvm-svn: 337204