scons: More narrowly target -Wno-self-assign.

This flag was necessary because of self assignments in the ISA parser
where self assignments are often hints to the parser itself, and in one
case because a pybind-ism used to attach the -= operator looked like a
self assignment.

This change narrows the scope of the flag that disables this warning to
only files generated by the ISA parser, and the single file in the
systemc code which uses that operator overload.

Change-Id: Ib64fc72e46f894cba9064afcdbdcc5859c30e745
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40952
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

SConstruct

@@ -375,10 +375,8 @@ elif main['CLANG']:
               'Installed version:', main['CXXVERSION'])
 
     # clang has a few additional warnings that we disable, extraneous
-    # parantheses are allowed due to Ruby's printing of the AST,
-    # finally self assignments are allowed as the generated CPU code
-    # is relying on this
-    main.Append(CCFLAGS=['-Wno-parentheses', '-Wno-self-assign'])
+    # parantheses are allowed due to Ruby's printing of the AST.
+    main.Append(CCFLAGS=['-Wno-parentheses'])
     conf.CheckCxxFlag('-Wno-c99-designator')
     conf.CheckCxxFlag('-Wno-defaulted-function-deleted')