arch: Move many of the generic files outside an NULL guard.

These files can be compiled successfully even if the ISA is the NULL
ISA.

Change-Id: I67133ea674f678f33b0aa1ef55af719f2869241d
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/34169
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/generic/SConscript

@@ -39,12 +39,6 @@
 Import('*')
 
 Source('htm.cc')
-
-if env['TARGET_ISA'] == 'null':
-    Return()
-
-Source('decode_cache.cc')
-Source('decoder.cc')
 Source('mmu.cc')
 
 SimObject('BaseInterrupts.py')
@@ -54,3 +48,9 @@ SimObject('BaseTLB.py')
 SimObject('ISACommon.py')
 
 DebugFlag('TLB')
+
+if env['TARGET_ISA'] == 'null':
+    Return()
+
+Source('decode_cache.cc')
+Source('decoder.cc')