[llvm] [Object,ELF] Implement PN_XNUM extension for program headers (PR #162288)

[James Henderson via llvm-commits]

https://github.com/jh7370 approved this pull request.

LGTM, on the proviso that we add testing for the new program header code in a follow-up PR, e.g. when updating llvm-readobj or another tool.

https://github.com/llvm/llvm-project/pull/162288