[libunwind] [libunwind] Use consistent indentation in `__libunwind_config.h` (PR #152861)

[Michał Górny via cfe-commits]

mgorny wrote:

> Should we maybe just set `IndentPPDirectives: AfterHash`? We already do that in libc++, libc++abi and (I think) compiler-rt.

I suppose it would make sense to me, but I don't feel like I'm the person to make that decision.

https://github.com/llvm/llvm-project/pull/152861