scons: Use tags to gate ISA files and not env['TARGET_ISA'].

Change-Id: Ib81a4c570fbb050fa7d82919edacfed004c6800e
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/50336
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>

src/dev/mips/SConscript

@@ -28,12 +28,10 @@
 
 Import('*')
 
-if env['TARGET_ISA'] == 'mips':
-    SimObject('Malta.py')
+SimObject('Malta.py', tags='mips isa')
 
-    DebugFlag('Malta')
-
-    Source('malta.cc')
-    Source('malta_cchip.cc')
-    Source('malta_io.cc')
+DebugFlag('Malta', tags='mips isa')
 
+Source('malta.cc', tags='mips isa')
+Source('malta_cchip.cc', tags='mips isa')
+Source('malta_io.cc', tags='mips isa')