[yaml2obj][ELF] Add support for program headers

This change adds basic support for program headers.

I need to do some testing which requires generating program headers but
I can't use ld.lld or clang to produce programs that have headers. I'd
also like to test some strange things that those programs may never
produce.

Patch by Jake Ehrlich

Differential Revision: https://reviews.llvm.org/D35276

llvm-svn: 308520