[clang] [Fuchsia] Support PGO (PR #128680)

[Petr Hosek via cfe-commits]

petrhosek wrote:

> LGTM. This seems to work locally for me, but I haven't tested w/ cross compiling. Is that still potentially an issue?

It's not addressed since that's going to require more changes, I assume we'll disable PGO on cross-compiling builders by not setting `FUCHSIA_ENABLE_PGO`.

https://github.com/llvm/llvm-project/pull/128680