Auto-generate arch/foo.hh "switch headers" in scons.

SConscript:
    Include new arch/SConscript file.
arch/isa_specific.hh:
    Get rid of unnecessary ISA_INCLUDE() macro and other
    things that were used only for that purpose.
build/SConstruct:
    Move list of ISAs to env var ALL_ISA_LIST.

--HG--
extra : convert_revision : 612c7ee4279d57209662be88dc577d80fdbd692c

SConscript

@@ -346,6 +346,8 @@ arch_source = SConscript('arch/%s/SConscript' % env['TARGET_ISA'],
 # Add a flag defining what THE_ISA should be for all compilation
 env.Append(CPPDEFINES=[('THE_ISA','%s_ISA' % env['TARGET_ISA'].upper())])
 
+SConscript('arch/SConscript', exports = 'env', duplicate = False)
+
 # Set up complete list of sources based on configuration.
 sources = base_sources + arch_source