PR d/90064
libphobos: Merge upstream phobos 428460ddd

Defines growDownwards on SPARC64, initial patch by Rainer Orth.

Backports another fix to std.process, allowing permissions tests to be
skipped when running as root.

Reviewed-on: https://github.com/dlang/phobos/pull/6962

From-SVN: r270483