games/stonesoup: Fix build on armv6/armv7.

Fix build on armv6/armv7 by removing a misguided OS-detection case.
Fix build with WRKDIR inside a git-maintained ports tree.
Turn the custom README into a pkg-message for better visibility.
Take over maintainership of this unmaintained port.

PR: 259499
(cherry picked from commit 5304a1914e4dfa3da4cb6d54932ff2f0d5e32f89)