By default, the --stderr-file and --stdout-file arguments were
directing the simulator output to files named "simerr" and
"simout" respectively if an output redirect was requested.
A small annoyance is these files lack an extension meaning programs
refuse to open them, or to do so without some additional effort. On
many systems they are assumed to scripts.
This patch adds the .txt extension to both, thus clearly indicating
to other programs these are text files and can be opened to be read
as such.
Change-Id: Iff5af4a9e6966b4467d005a029dbf401099fbd35
391f62b213