Default platform to Linux for ELF files in Util::EXE 

Creating ELF files via msfvenom was hitting an exception in `Util::EXE` when trying to compare the `String` platform passed in from `PayloadGenerator` with a `Platform` object.

See #2950 

## Verification steps
 - [ ] `./msfvenom -f elf -p linux/x86/shell_reverse_tcp LHOST=... > shell.elf`