math/py-fastcluster: fix build on powerpc* with LLVM 14 by forcing 15

Assertion failed: ((CGF.CurFuncDecl == nullptr || CGF.Builder.getIsFPConstrained() || isa<CXXConstructorDecl>(CGF.CurFuncDecl) || isa<CXXDestructorDecl>(CGF.CurFuncDecl) || (NewExceptionBehavior == llvm::fp::ebIgnore && NewRoundingBehavior == llvm::RoundingMode::NearestTiesToEven)) && "FPConstrained should be enabled on entire function"), function ConstructorHelper, file /usr/local/poudriere/jails/main-powerpc64/usr/src/contrib/llvm-project/clang/lib/CodeGen/CodeGenFunction.cpp, line 163.