configs: Add --redirects for syscall emulation

This is the first step towards being able to run dynamically linked
applications when the guest ISA != than host ISA.

(Like running a arm application on x86)

By using the --redirects command line option it is possible to specify
via CLI a set of path redirections to be used in SE mode.

This is needed when running a dynamically linked binary in
SE mode in a guest ISA different than the host. The linker will look
for SOs (e.g. libc.so) in /lib/, but will only find the host libraries.
With this option we can redirect to the guest toolchain/file system.

Usage:

gem5.opt [example script]
    --redirects /dir1=/path/to/host/dir1 \
    --redirects /dir2=/path/to/host/dir2

Change-Id: I558838be2ad6802891707e9a1cc454786859db15
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23065
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/common/FileSystemConfig.py

@@ -1,3 +1,15 @@
+# Copyright (c) 2019 ARM Limited
+# All rights reserved.
+#
+# The license below extends only to copyright in the software and shall
+# not be construed as granting a license to any other intellectual
+# property including but not limited to intellectual property relating
+# to a hardware implementation of the functionality of the software
+# licensed hereunder.  You may use the software subject to the license
+# terms below provided that you ensure that this notice is replicated
+# unmodified and in its entirety in all distributions of the software,
+# modified or unmodified, in source code or in binary form.
+#
 # Copyright (c) 2015 Advanced Micro Devices, Inc.
 # All rights reserved
 #
@@ -206,6 +218,14 @@ def _redirect_paths(options):
                       RedirectPath(app_path = "/tmp",
                           host_paths = ["%s/fs/tmp"  % m5.options.outdir])]
 
+    # Setting the redirect paths so that the guest dynamic linker
+    # can point to the proper /lib collection (e.g. to load libc)
+    redirects = getattr(options, 'redirects', [])
+    for redirect in redirects:
+        app_path, host_path = redirect.split("=")
+        redirect_paths.append(
+            RedirectPath(app_path = app_path, host_paths = [ host_path ]))
+
     chroot = getattr(options, 'chroot', None)
     if chroot:
         redirect_paths.append(