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
Petr Hosek