emulators/mednafen: fix build on powerpc64 elfv2

Switch to GCC on powerpc64, clang has already reported issue:
Assertion failed: (!isSigned && "Legalize cannot Expand SINT_TO_FP for i64 yet"), function ExpandLegalINT_TO_FP, file /usr/src/contrib/llvm-project/llvm/lib/CodeGen/SelectionDAG/LegalizeDAG.cpp, line 2420.