sim: Disable gzip compression for writefile pseudo instruction

sim: Disable gzip compression for writefile pseudo instruction

The writefile pseudo instruction uses OutputDirectory::create and
OutputDirectory::openFile to create the output files. However, by
default these will check the file extention for .gz, and create a gzip
compressed stream if the file ending matches. When writing out files,
we want to write them out exactly as they are in the guest simulation,
and never want to compress them with gzio. Additionally, this causes
m5 writefile to fail when checking the error flags for the output
steam.

With this patch we add an additional no_gz argument to
OutputDirectory::create and OutputDirectory::openFile which allows us
to override the gzip compression.  Therefore, for m5 writefile we
disable the filename check, and always create a standard ostream.