[llvm-objdump] Create fake sections for a ELF core file

The linux perf tools use /proc/kcore for disassembly kernel functions.
Actually it copies the relevant parts to a temp file and then pass it to
objdump. But it doesn't have section headers so llvm-objdump cannot
handle it.

Let's create fake section headers for the program headers. It'd have a
single section for each segment to cover the entire range. And for this
purpose we can consider only executable code segments.

With this change, I can see the following command shows proper outputs.

perf annotate --stdio --objdump=/path/to/llvm-objdump

Differential Revision: https://reviews.llvm.org/D128705