Support for OpenVMS host supports

[Paolo Bonzini]

I stumbled in this while looking at how x-* host files are used.  There 
are two files in this configuration that "must be compiled with DEC C".

One is vcrt0.o, which has about 5 lines of executable code.  This makes 
me think that it would be best if someone with access to the OS would 
compile it, so that we can put assembly-language source code for it.

The other is pcrt0.o, which AFAICT had a syntax error since its inception:

  48001     kenner __main (arg1, arg2, arg3, image_file_desc, arg5, arg6)
  48001     kenner      void *arg1, *arg2, *arg3;
  48001     kenner      void *image_file_desc;
  48001     kenner      void *arg5, *arg6)

So, the question is: do we care about this target?  Maybe AdaCore has 
patches to fix it?

Paolo