Make TmpFs independent of DynFlags

This is small step towards #19877 (closed). We want to make the Loader/Linker interface more abstract to be easily reused (i.e. don't pass it DynFlags) but the system linker uses TmpFs which required a DynFlags value to get its temp directory. We explicitly pass the temp directory now. Similarly TmpFs was consulting the DynFlags to decide whether to clean or: this is now done by the caller in the driver code.