[LLD][ELF] Fix --check-dynamic-relocations for 32-bit targets

OutputSection::checkDynRelAddends() incorrectly reports an internal
linker error for large addends on 32-bit targets. This is caused by the
lack of sign extension in DynamicReloc::computeAddend() for 32-bit
addends.

Differential Revision: https://reviews.llvm.org/D149347