sim-se: add a faux-filesystem

This change introduces the concept of a faux-filesystem.
The faux-filesystem creates a directory structure in m5out
(or whatever output dir the user specifies) where system calls
may be redirected.

This is useful to avoid non-determinism when reading files
with varying path names (e.g., variations from run-to-run if
the simulation is scheduled on a cluster where paths may change).

Also, this changeset allows circumventing host pseudofiles which
have information specific to the host processor (such as cache
hierarchy or processor information). Bypassing host pseudofiles
can be useful when executing runtimes in the absence of an
operating system kernel since runtimes may try to query standard
files (i.e. /proc or /sys) which are not relevant to an
application executing in syscall emulation mode.

Change-Id: I90821b3b403168b904a662fa98b85def1628621c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/12119
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs/example/se.py

@@ -64,6 +64,7 @@ from common import CacheConfig
 from common import CpuConfig
 from common import BPConfig
 from common import MemConfig
+from common.FileSystemConfig import redirect_paths, config_filesystem
 from common.Caches import *
 from common.cpu2000 import *
 
@@ -241,6 +242,9 @@ for i in range(np):
 
     system.cpu[i].createThreads()
 
+system.redirect_paths = redirect_paths(os.path.expanduser(options.chroot))
+config_filesystem(options)
+
 if options.ruby:
     Ruby.create_system(options, False, system)
     assert(options.num_cpus == len(system.ruby._cpu_ports))