llvm-readobj: Fix addend in relocations for android packed format

If a relocation group doesn't have the RELOCATION_GROUP_HAS_ADDEND_FLAG set, then this implies the group's addend equals zero.
In this case android packed format won't encode an explicit addend delta, instead we need to set Addend, the "previous addend" variable, to zero by ourself.

Patch by Yi-Yo Chiang!

Differential Revision: https://reviews.llvm.org/D50601

llvm-svn: 339799