Custom loader plugin decompilation output is nonsense

First all of all, kudos for the custom loader plugin support and `IDA 7.x` support but I'm still not happy with the decompilation output.

Given a function with the following assembly code:

```
.text:02008C5C __PerformSyscall:
.text:02008C5C li r0, 0x12
.text:02008C60 sc
.text:02008C64 blr
```

If I run the `retdec` decompiler on it, it produces the following output:
```
int32_t __PerformSyscall(void) {
// 0x2008c5c
return 0;
}
```

This is false because:
The function does not define a return value (`r3` is not set) hence the return type is `void`. Furthermore, the syscall (`sc` instruction) is not translated at all. A proper decompilation (in my opinion) may look like the following:
```
void __PerformSyscall(void) {
asm volatile
(
"li r0, 0x12\n"
"sc"
);
}
```

Since the syscall instruction does not exist in `C`, it has to use inline assembly.

Note that this is just one of many failing examples (all functions seem to just translate to `return 0`). In fact, `retdec` seemingly cannot deal with the custom plugin at all even though the `IDA` database holds all necessary information to perform the decompilation. Therefore, why does it not operate correctly on the database? I would either expect an error message if something went wrong with reading the right data or a "proper" decompilation. Not a default "no-op" decompilation like this.

Note that this is a follow-up issue to the following issue: https://github.com/avast-tl/retdec-idaplugin/issues/3 (Check it out for more information or to download the custom plugin as well as an example binary)