We need some help to finally fix Darwin ppc64 assembler so that not just build succeeds but also tests pass

In a couple of recent PRs I have made some fixes to Darwin PPC assembler here (nothing big, just fixing which was actually breaking the build), which fixed _building_ of `context` and made it _somewhat_ functional (in a sense that certain dependencies that require `context` now build successfully, `folly` and friends being the case).
However, tests do not pass: https://github.com/macports/macports-ports/pull/16407#issuecomment-1304974991

From earlier PRs by @kernigh and @DaoWen for PPC ELF it looks like the issue is quite non-trivial and requires a thorough understanding of both what Boost is doing and the _magic code_ :)
Unfortunately, my knowledge of PPC assembler is rather limited and does not suffice here. I am not going to drop the case, since we really want this fixed properly, but any advice or help will be greatly appreciated.

_This has been broken apparently forever – can we fix it now?_ :)

(Considering a recent case with Boost shared pointer (https://github.com/boostorg/smart_ptr/issues/105), I cannot be entirely sure that it is assembler which is to blame and not again some wrong alignment, but probably it is assembler, given that it was also broken for *BSD.)

P. S. For the record, alternative version of PPC code for `context` from the following repo also fails, at least with Fibonacci test: https://github.com/twlostow/libcontext

P. P. S. @iains I am aware you are very busy atm, so please feel free to completely ignore the issue. But still tagging you as the person who understands Darwin magic code best.