Serialize builds of `rts` and `system-cxx-std-lib`

Fixes #22099 (closed)

I cannot say for sure that this will fix the linked issue, but I explained why I believe this fixes the root problem in:

#22099 (comment 461386)

… and when we applied this patch internally the problem appeared to disappear for a few builds in a row with a large number of cores.

The short summary of the above comment is that the Makefile serializes the package registrations using ghc-pkg, with the exception of two packages (rts and system-cxx-std-lib) and we expect that this one instance of parallelism that was still left in the Makefile was the root cause of the flaky build failure.

This fix would be consistent with two known things about the failure:

• The build failure only occurs for parallel Make builds

  … and these are the only two ghc-pkg calls that run concurrently

• The error first appeared in the GHC 9.4 branch and was absent in GHC 9.2

  … and GHC 9.4 was the first release that included system-cxx-std-lib (as far as I know), which would explain why this problem didn't happen before.

The change here forces the two packages to be registered in sequence.

This problem will of course go away in GHC 9.6 by virtue of dropping the make-based build, but since the GHC 9.4 branch is supported and active we wanted to upstream this patch to the 9.4 development branch.