[yaml2obj][ELF] Add support for setting alignment in program headers

[yaml2obj][ELF] Add support for setting alignment in program headers

Sometimes program headers have larger alignments than any of the
sections they contain. Currently yaml2obj can't produce such files. A
bug recently appeared in llvm-objcopy that failed in such a case. I'd
like to be able to add tests to llvm-objcopy for such cases.

This change adds an optional alignment parameter to program headers that
will be used instead of calculating the alignment.

Differential Revision: https://reviews.llvm.org/D39130

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@317139 91177308-0d34-0410-b5e6-96231b3b80d8