scons: Fix an error about an unrecognized compiler.

join was being passed a series of strings to join as separate
arguments like os.path.join, but it expects to get them as members of a
single sequence.

Change-Id: Id88ce4e9c5400f256a1af6351b4a964af0036b72
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/25203
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -389,7 +389,7 @@ if main['GCC'] or main['CLANG']:
                          '-Wno-error=deprecated',
                         ])
 else:
-    error('\n'.join(
+    error('\n'.join((
           "Don't know what compiler options to use for your compiler.",
           "compiler: " + main['CXX'],
           "version: " + CXX_version.replace('\n', '<nl>') if
@@ -400,7 +400,7 @@ else:
           "",
           "If you are trying to use a compiler other than those listed",
           "above you will need to ease fix SConstruct and ",
-          "src/SConscript to support that compiler."))
+          "src/SConscript to support that compiler.")))
 
 if main['GCC']:
     # Check for a supported version of gcc. >= 4.8 is chosen for its