configs: Generalize FileSystemConfig for non se.py

This patch updates the FileSystemConfig so it works with more kinds of
config scripts (e.g., the Learning gem5 scripts).

There are 4 main changes:
- Added system as a parameter to the config_filesystem function so the
function can search the system for the number of CPUs instead of relying
on options from Options.py
- Instead of calling redirect_paths everywhere config_filesystem is
used, now it is implicitly called.
- Cleaned up the Ruby scripts a bit to remove redundant calls to
config_filesystem
- Added a config_filesystem call to the Ruby Learning gem5 script
(currently the only Learning gem5 script that requires it).

In the future, I think it would be better to move the config_filesystem
call into simulate.py, probably into the instantiate function. I tried to
use the per-CPU configuration parameters instead of options from
Options.py, but that's not possible until after the SimObject params
have been finalized in instantiate.

Change-Id: Ie6501a7435cfb3ac9d2b45be3722388b34063b1e
Signed-off-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18848
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Tested-by: kokoro <noreply+kokoro@google.com>

config, sim-se: bugfix for 54c77aa0

The NULL ISA does not have some members for the options
class which are referenced by the FileSystemConfig
code.

Create default values for the members so that the
simulation does not fail during the configuration phase.

Change-Id: Ie65bf0e5550c964eae42d1df4c36c2c5bc4ea703
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/18748
Reviewed-by: Daniel Carvalho <odanrc@yahoo.com.br>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>

configs: faux-filesystem fix w/ ruby in se mode

These changes are needed so that the config scripts
can report cache hierarchy information to the faux
filesystem.

This is useful for the ROCm runtime when it reads
psuedofiles from the host filesytem from "/proc".

Change-Id: Iad3e6c088d47c9b93979f584de748367eae8259b
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/12121
Reviewed-by: Brandon Potter <Brandon.Potter@amd.com>
Maintainer: Brandon Potter <Brandon.Potter@amd.com>
Tested-by: kokoro <noreply+kokoro@google.com>

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>