cpu: Restructure BTB

- A new abstract BTB class is created to enable different BTB
  implementations. The new BTB class gets its own parameter
  and stats.
- An enum is added to differentiate branch instruction types.
  This enum is used to enhance statistics and BPU management.
- The existing BTB is moved into `simple_btb` as default.
- An additional function is added to store the static instruction in
  the BTB. This function is used for the decoupled front-end.
- Update configs to match new BTB parameters.

Change-Id: I99b29a19a1b57e59ea2b188ed7d62a8b79426529
Signed-off-by: David Schall <david.schall@ed.ac.uk>

src/python/gem5/prebuilt/riscvmatched/riscvmatched_core.py

@@ -95,7 +95,7 @@ class U74FUPool(MinorFUPool):
 
 
 class U74BP(TournamentBP):
-    BTBEntries = 32
+    btb = SimpleBTB(numEntries=32)
     RASSize = 12
     localHistoryTableSize = 4096  # is 3.6 KiB but gem5 requires power of 2
     localPredictorSize = 16384