[llvm-objcopy] Cache gnu_debuglink's target CRC

.gnu_debuglink section contains information regarding file with
debugging symbols, identified by its CRC32. This target file is not
intended to ever change or it would invalidate the stored checksum, yet
the checksum is calculated over and over again for each of the objects
inside the archive, usually hundreds of times.

This patch precomputes the CRC32 of the target once and then reuses the
value where required, saving lots of redundant I/O.

The error message reported should stay the same, although now it might
be reported earlier.

Reviewed by: jhenderson, jakehehrlich, MaskRay

Differential Revision: https://reviews.llvm.org/D61343

Patch by Michal Janiszewski

llvm-svn: 360661