[llvm-elfabi] Support ELF file that lacks .gnu.hash section

Before this change, when reading ELF file, elfabi determines number of
entries in .dynsym by reading the .gnu.hash section. This change makes
elfabi read section headers directly first. This change allows elfabi
works on ELF files which do not have .gnu.hash sections.

Differential Revision: https://reviews.llvm.org/D93362