scons: arch: Generalize the switching header code.

Factor out the ISA ness of the switching header generating function. Also
turn it into a SCons builder which builds a single header, and a wrapping
method which uses the builder on a group of header files which all target
the same subdirectory.

Change-Id: I87705f97b6ebd9baebd4ebcfea19cc1218a64ad0
Reviewed-on: https://gem5-review.googlesource.com/2983
Reviewed-by: Curtis Dunham <curtis.dunham@arm.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>

src/arch/SConscript

@@ -43,8 +43,8 @@ Import('*')
 #
 #################################################################
 
-# List of headers to generate
-isa_switch_hdrs = Split('''
+env.SwitchingHeaders(
+    Split('''
         decoder.hh
         interrupts.hh
         isa.hh
@@ -63,10 +63,8 @@ isa_switch_hdrs = Split('''
         types.hh
         utility.hh
         vtophys.hh
-        ''')
-
-# Set up this directory to support switching headers
-make_switching_dir('arch', isa_switch_hdrs, env)
+        '''),
+    env.subst('${TARGET_ISA}'))
 
 if env['BUILD_GPU']:
     gpu_isa_switch_hdrs = Split('''