[llvm] [Object,ELF] Implement PN_XNUM extension for program headers (PR #165284)

[Fangrui Song via llvm-commits]

MaskRay wrote:

Do we have a test for `PN_XNUM`? It's ok to use PN_XNUM when there are not that many program headers (to prevent creating a large output file in the test directory)

https://github.com/llvm/llvm-project/pull/165284