make BTB fully associative; don't use it for JALR

JALR created a long path from the ALU in execute stage
to an address comparator to the next-PC mux.  the benfit
was close to nil, anyway.

rocket/src/main/scala/consts.scala

@@ -22,6 +22,7 @@ object Constants
   val PC_PCR  = UFix(4, 3);
   val PC_WB   = UFix(5, 3);
   val PC_EVEC = UFix(6, 3);
+  val PC_JR   = UFix(7, 3);
 
   val KF_Y  = UFix(1, 1);
   val KF_N  = UFix(0, 1);