drfuzz: fix calling convention flags and move them to drwrap

The calling convention flags in drfuzz are all messed up, e.g. there is no such thing as `fastcall` on any 64-bit platform. Since drfuzz relies on drwrap for everything related to the calling convention, the simplest solution is to move the flags to drwrap, after fixing them (xref [DR i#1772](https://github.com/DynamoRIO/dynamorio/issues/1772)). There can be a simple pass-through for drwrap flags on `drfuzz_fuzz_target()`.