scons: fix build failed caused by Non-ASCII directory path

This path fix build failed when gem5's path contains Non-ASCII
characters.

build_tools/marshal.py embed "abspath" and "modpath" into the
generated c++ file. However, if either of them contains Non-ASCII
characters, marshal.py will throw an exception.

This can be fixed if we set a suitable $LANG for gem5py. For
example, we can simply set $LANG=en_US.UTF-8 in src/SConscript
to fix this.

This patch fixes this bug by passing the host's $LANG environment
variable to gem5py. Because for users who may use Non-ASCII
characters, their $LANG is generally set to their native language
(e.g. zh_CN.UTF-8, en_US.UTF-8, etc.), which contains Non-ASCII
characters. Also, it is more reasonable to use the host $LANG than
to directly hard-code the $LANG used by gem5py into a language
that supports non-ASCII characters.

Change-Id: Ia0343cbd6995af3a9e43bf217ad3f186d5633fed
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/58369
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

site_scons/gem5_scons/defaults.py

@@ -48,7 +48,7 @@ def EnvDefaults(env):
                      'LIBRARY_PATH', 'PATH', 'PKG_CONFIG_PATH', 'PROTOC',
                      'PYTHONPATH', 'RANLIB', 'TERM', 'PYTHON_CONFIG',
                      'CCFLAGS_EXTRA', 'GEM5PY_CCFLAGS_EXTRA',
-                     'GEM5PY_LINKFLAGS_EXTRA', 'LINKFLAGS_EXTRA'])
+                     'GEM5PY_LINKFLAGS_EXTRA', 'LINKFLAGS_EXTRA', 'LANG'])
 
     use_prefixes = [
         "ASAN_",           # address sanitizer symbolizer path and settings