gcc-15 bugs, pile 1

About 4 months have passed since gcc-14.1.0 release. Around the same time gcc-15 development has started and a few major changes were merged into the master development branch. summary This time I waited to collect about 20 bug reports I encountered: c++/114933 : mcfgthread-1.6.1 type check failure. Ended up being mcfgthread bug caused by stronger gcc checks. tree-optimization/114872 : sagemath…