graphics/dcmtk: use LLVM 15 on powerpc* when 14 is in the system

LLVM 14 crashes:
Assertion failed: (all_of(I->users(), [&InsertedSet](Value *U) { return InsertedSet.contains(cast<Instruction>(U)); }) && "removed instruction should only be used by instructions inserted " "during expansion"), function cleanup, file /usr/local/poudriere/jails/powerpc64-140/usr/src/contrib/llvm-project/llvm/lib/Transforms/Utils/ScalarEvolutionExpander.cpp, line 2798.