PPC64: Implement post-call NOPs

Post-call NOPs encode the offset from a compiled call to the CodeBlob the compiled call belongs to. This allows for quick CodeBlob look-up when iterating compiled stack frames.

[~rpressler] commented on the review of JDK-8287567: "The NOPs make a big difference for various stack walking operations, and while we did see some negative effects, they're probably very minor. However, because they're only generated in JDK 19 if preview is enabled, we can perform a deeper performance analysis later."

Also make use of the quick lookup when reconstructing the back chain during fast thaw (see `Thaw<ConfigT>::patch_caller_links()`)