Fix cfloat return on x86_64: only perform ABI transformation for non-extern(D) functions. There's no need to waste cycles with extern(D), which we get to define ourselves. Fixes tests/mini/asm8.d. (Since the asm abiret code already assumed {xmm0, xmm1} returns)

// D runtime support helpers

bool LLVM_D_InitRuntime();
void LLVM_D_FreeRuntime();

llvm::Function* LLVM_D_GetRuntimeFunction(llvm::Module* target, const char* name);

llvm::GlobalVariable* LLVM_D_GetRuntimeGlobal(llvm::Module* target, const char* name);