scons: Add '-lstdc++fs' to LIBS env when GCC version < 9

This commit is in response to a failure in the compiler tests:
https://jenkins.gem5.org/job/compiler-checks/570/

GCC versions <9 failed to compile gem5 with the following error:

```
socket.cc:(.text+0x32d5): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
socket.cc:(.text+0x32e5): undefined reference to `std::filesystem::create_directory(std::filesystem::__cxx11::path const&, std::error_code&)'
socket.cc:(.text+0x3370): undefined reference to `std::filesystem::current_path[abi:cxx11](std::error_code&)'
socket.cc:(.text+0x33cc): undefined reference to `std::filesystem::__cxx11::path::_M_split_cmpts()'
socket.cc:(.text+0x33d7): undefined reference to `std::filesystem::current_path(std::filesystem::__cxx11::path const&, std::error_code&)'
socket.cc:(.text+0x3468): undefined reference to `std::filesystem::current_path(std::filesystem::__cxx11::path const&, std::error_code&)'
```

This was due to this patch:
https://gem5-review.googlesource.com/c/public/gem5/+/69164 which
utilized `std::filesystem`. Prior to to GCC v9, explicit linking with
the stdc++fs library is required due to `std::filesystem` being an
experimental implementation prior to GGC v9.

Change-Id: I584e29f100cb59b40bd155c212e1814f6d8fbb99
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/69597
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>

SConstruct

@@ -483,6 +483,17 @@ for variant_path in variant_paths:
             '-fno-builtin-malloc', '-fno-builtin-calloc',
             '-fno-builtin-realloc', '-fno-builtin-free'])
 
+        if compareVersions(env['CXXVERSION'], "9") < 0:
+            # `libstdc++fs`` must be explicitly linked for `std::filesystem``
+            # in GCC version 8. As of GCC version 9, this is not required.
+            #
+            # In GCC 7 the `libstdc++fs`` library explicit linkage is also
+            # required but the `std::filesystem` is under the `experimental`
+            # namespace(`std::experimental::filesystem`).
+            #
+            # Note: gem5 does not support GCC versions < 7.
+            env.Append(LIBS=['stdc++fs'])
+
     elif env['CLANG']:
         if compareVersions(env['CXXVERSION'], "6") < 0:
             error('clang version 6 or newer required.\n'