arch,dev,gpu-compute,sim: Rename isa_traits.hh page_size.hh.

The only thing left in isa_traits.hh are two constants, one for the
number of bytes in a page, and one for how far to shift an address to
get the page number. To make it clear that this is the only thing
isa_traits.hh should be used for from this point forward (until it is
entirely eliminated), this change renames it to the much less generic
page_size.hh.

Also, because isa_traits.hh used to have *much* more stuff in it, it was
included in a lot of places it didn't need to be. This change also
clears out all these legacy includes while updating the actually needed
ones to the new name.

Change-Id: I939b01b117c53d620b6b0a98982f6f21dc2ada72
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/40179
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/SConscript

@@ -59,8 +59,8 @@ env.SwitchingHeaders(
     Split('''
         decoder.hh
         isa.hh
-        isa_traits.hh
         locked_mem.hh
+        page_size.hh
         registers.hh
         remote_gdb.hh
         types.hh