Fix use of the getOpenFile api.

The gold plugin was passing the desired map size as the file size. This was
working for two reasons:
* Recent version of gold provide the get_view callback, so this code was not
  used.
* In older versions, getOpenFile was called, but the file size is never used
  if we don't require null terminated buffers and map size defaults to the
  file size.

Thanks to Eli Bendersky for noticing this.

I will try to make this api a bit less error prone.

llvm-svn: 186978