scons: Enable the clang++ and clang tools.

If these tools aren't enabled and CXX isn't set, scons will look for
generically named compiler aliases like cc and c++. These will generally
work, but if scons knows that the compiler is specifically clang, it
will set the CXXCONFIG variable we can use to do compiler version
checking.

Because scons blindly forces a tool into use if you specify it, we need
to use the FindTool method which will check a list of tools and add the
first one that it actually finds.

Change-Id: Ie6bebb8eab531989575c878bee07189541756d2c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41673
Reviewed-by: Earl Ou <shunhsingou@google.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -94,6 +94,7 @@ from re import match
 import SCons
 import SCons.Node
 import SCons.Node.FS
+import SCons.Tool
 
 from m5.util import compareVersions, readCommand, readCommandWithReturn
 
@@ -137,6 +138,9 @@ import gem5_scons
 
 main = Environment(tools=['default', 'git', TempFileSpawn])
 
+main.Tool(SCons.Tool.FindTool(['gcc', 'clang'], main))
+main.Tool(SCons.Tool.FindTool(['g++', 'clang++'], main))
+
 from gem5_scons.util import get_termcap
 termcap = get_termcap()