[llvm-readobj] - Do not crash when an invalid .eh_frame_hdr is dumped using --unwind.

When the p_offset/p_filesz of the PT_GNU_EH_FRAME is invalid
(e.g larger than the file size) then llvm-readobj might crash.

This patch fixes the issue. I've introduced `ELFFile<ELFT>::getSegmentContent`
method, which is very similar to `ELFFile<ELFT>::getSectionContentsAsArray` one.

Differential revision: https://reviews.llvm.org/D80380