[llvm-objdump] - Introduce getRelocsMap() helper. NFCI.

Currently disassembleObject() is a ~550 lines length function.
This patch extracts the code that creates a section->their relocation
mapping into a new helper function to simplify/reduce it a bit.

Differential revision: https://reviews.llvm.org/D57019

llvm-svn: 351824