arch,sim: Promote the m5ops_base param to the System base class.

This mechanism is shared between ARM and x86, even if x86 has a typical
address range it choses to use. By moving this to the base class, it's
now possible for anybody to find out where the m5 ops are, and no ISA
specific assumptions need to be made.

Because the x86 address is well known, it's set in the x86 System
subclass as the default.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: Ifdb9f5cd1ce38b3c4dafa7566c50f245f14cf790
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23180
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/ArmSystem.py

@@ -93,10 +93,6 @@ class ArmSystem(System):
     semihosting = Param.ArmSemihosting(NULL,
         "Enable support for the Arm semihosting by settings this parameter")
 
-    m5ops_base = Param.Addr(0,
-        "Base of the 64KiB PA range used for memory-mapped m5ops. Set to 0 "
-        "to disable.")
-
     dtb_filename = Param.String("",
         "File that contains the Device Tree Blob. Don't use DTB if empty.")