[llvm-readelf] - Rework how we parse the .dynamic section.

This is a result of what I found during my work on https://bugs.llvm.org/show_bug.cgi?id=41679.

Previously LLVM readelf took the information about .dynamic section
from its PT_DYNAMIC segment only. GNU tools have a bit different logic.
They also use the information from the .dynamic section header if it is available.
This patch changes the code to improve the compatibility with the GNU Binutils.

Differential revision: https://reviews.llvm.org/D61937

llvm-svn: 361165