Add a wrapper script to run a simple_system binary under Spike

The main point of this is that it's somewhere we can store what it
calls "ss_opts": the options that we must pass to Spike to tell it the
memory layout of the simple_system environment.

While we're at it, I've tried to make this reasonably pleasant to use.
So you can do

   spike-simple-system.sh <elf-file>

just to run the thing. To pass stuff to Spike without the
bash script getting in the way, use '--':

   spike-simple-system.sh -- my option -h --here <elf-file>

examples/simple_system/spike-simple-system.sh

@@ -0,0 +1,108 @@
+#!/bin/bash
+
+set -u
+set -e
+
+# This is a simple bash script to allow you to run a binary compiled
+# for the simple_system environment using Spike.
+#
+
+error() {
+    echo >&2 "$@"
+    exit 1
+}
+
+usage () {
+    echo >&2 "Usage:  spike-simple-system [-h] [-v] [-n] [--] [options] <elf>"
+    echo >&2
+    echo >&2 "  Where <elf> is the path to an ELF binary that was"
+    echo >&2 "  compiled for the simple_system environment. Any arguments "
+    echo >&2 "  in <options> will be passed to Spike after those to set up "
+    echo >&2 "  the environment for simple_system tests."
+    echo >&2
+    echo >&2 "  If the -v argument is passed, the Spike command will be "
+    echo >&2 "  echoed to stderr before it runs. If the -n argument is passed "
+    echo >&2 "  (which implies -v), we don't actually run the command."
+    echo >&2
+    echo >&2 "  This will write the Spike instruction log to stderr."
+
+    exit $1
+}
+
+declare -a opts
+opts=()
+
+verbose=0
+dryrun=0
+
+at_start=1
+
+# We can be rather lazy when building our command line here, and don't
+# need to distinguish between [options] and <elf>, since they end up
+# appended to the Spike command line in the same order.
+while [ $# != 0 ]; do
+    if [ $at_start = 1 ]; then
+        case "$1" in
+            --help|-h)
+                usage 0
+                ;;
+
+            --verbose|-v)
+                verbose=1
+                ;;
+
+            --dry-run|-n)
+                dryrun=1
+                verbose=1
+                ;;
+
+            --)
+                at_start=0
+                ;;
+
+            *)
+                at_start=0
+                opts=("$1")
+                ;;
+        esac
+    else
+        opts+=("$1")
+    fi
+    shift
+done
+
+# If opts is empty, that's definitely an error (since the <elf>
+# parameter was compulsory). Moan here.
+if [ ${#opts[*]} = 0 ]; then
+    usage 1
+fi
+
+# Time to figure out how to call the Spike binary. If the user has set
+# SPIKE_PATH, we should obey that. Otherwise, we'll just assume that
+# they've got their PATH set up sensibly.
+if [ x"$SPIKE_PATH" = x ]; then
+    spike='spike'
+else
+    spike="$SPIKE_PATH/spike"
+fi
+
+# Here are the Spike options you need to run in a simple_system world.
+declare -a ss_opts
+ss_opts=(--isa=rv32imc
+         --log-commits
+         -l
+         -m0x10000:0x30000,0x100000:0x100000)
+
+cmd=("$spike" "${ss_opts[@]}" "${opts[@]}")
+
+if [ $verbose = 1 ]; then
+    # Echo the command that we're going to run in the same style as
+    # 'set -x' would.
+    echo >&2 + "${cmd[@]}"
+fi
+
+if [ $dryrun = 1 ]; then
+    exit 0
+fi
+
+exec "${cmd[@]}"