runtime_loader: Detect and trigger on PE binaries

* Previously PE binaries would trigger the "incorrectly
  executable" dialog. Now we get a special message for
  B_LEGACY_EXECUTABLE and B_UNKNOWN_EXECUTABLE
* Legacy at the moment is a R3 x86 PE binary. This could
  be extended to gcc2 binaries someday far, far, down the
  road though
* The check for legacy is based on a PE flag I see
  set on every R3 binary (that isn't set on dos ones)
* Unknown is something we know *is* an executable, but
  can't do anything with (such as an MSDOS or Windows
  application)
* No performance drops as we do the PE scan last
* Tested on x86 and x86_gcc2