cpu: Rename initiateHtmCmd to be more generic

To prepare for future CHI work, rename ExecContext::initiateHtmCmd to
ExecContext::initiateSpecialMemCmd

JIRA: https://gem5.atlassian.net/browse/GEM5-1097

Change-Id: I7c7aed8ec06a66d9023c14dba37eae42907df222
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/56598
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/cpu/checker/cpu.hh

@@ -242,7 +242,7 @@ class CheckerCPU : public BaseCPU, public ExecContext
     };
 
     Fault
-    initiateHtmCmd(Request::Flags flags) override
+    initiateSpecialMemCmd(Request::Flags flags) override
     {
         panic("not yet supported!");
         return NoFault;