Changed $*EXECUTABLE to $*EXECUTABLE_NAME that works

EXECUTABLE_NAME suits more cases than EXECUTABLE

```
$ perl6
> say $*EXECUTABLE
IO::Path </home/kamil/dev/panda/perl6>
> say $*EXECUTABLE_NAME
perl6
>


$ /home/kamil/rakudo/install/bin/perl6
> say $*EXECUTABLE
IO::Path </home/kamil/rakudo/install/bin/perl6>
>  $*EXECUTABLE_NAME
/home/kamil/rakudo/install/bin/perl6
>

$ perl6 --version
This is perl6 version 2014.04-187-gf1dd8a8 built on parrot 6.1.0 revision RELEASE_6_1_0
```