[AArch64] [Windows] Misc fixes for llvm-readobj -unwind.

Use getImageBase() helper to compute the image base. Fix various
offsets/addresses/masks so they're actually correct.

This allows decoding unwind info from DLLs, and unwind info from object
files containing multiple functions.

Differential Revision: https://reviews.llvm.org/D54015

llvm-svn: 346036