[ELF] - Fix for PR28976 - Corrupted section contents when using linker scripts

This is fix for PR28976.

Problem was that in scanRelocs, we computed relocation offset too early 
for case when linkerscript was used. Patch fixes the issue 
delaying the calculation.

Differential revision: https://reviews.llvm.org/D23655

llvm-svn: 279264