Rakefile: make bin directory

Sometimes ```./bin/``` doesn't exist because I removed or didn't copy/move it.
I think making sure it does, shouldn't be a big problem.
And the error message, if it doesn't, might be confusing for some people:
```
AR    build/host/lib/libmruby_core.lib
LD    build/host/bin/mrbc.exe
rake aborted!
No such file or directory - c:/libs/build/mruby-llvm/bin/mrbc.exe
c:/libs/build/mruby-llvm/Rakefile:42:in `block (2 levels) in <top (required)>
Tasks: TOP => default => all => c:/libs/build/mruby-llvm/bin/mrbc.exe
(See full trace by running task with --trace)
```