scons: Turn all global sticky variables into variant ones.

Saved option files are processed twice, once to load the value of the
EXTRAS variable, and then again once variables have been set up by
SConsopts.

Change-Id: Id8d49051c8fe684142a68259c470ed7c86bdab53
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56753
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/SConscript

@@ -57,6 +57,9 @@ Export(SourceFilter.factories)
 
 Import('*')
 
+if env['USE_EFENCE']:
+    env.Append(LIBS=['efence'])
+
 # Children need to see the environment
 Export('env')