Per ABI, only x1 and x5 should be treated as function returns

We were doing so for x3 and x7, as well, which could reduce performance
for compilers that happen to perform indirect jumps via t2 (x7).

src/main/scala/rocket/rocket.scala

@@ -603,7 +603,7 @@ class Rocket(implicit p: Parameters) extends CoreModule()(p) {
   io.imem.btb_update.valid := (mem_reg_replay && mem_reg_btb_hit) || (mem_reg_valid && !take_pc_wb && (((mem_cfi_taken || !mem_cfi) && mem_wrong_npc) || (Bool(fastJAL) && mem_ctrl.jal && !mem_reg_btb_hit)))
   io.imem.btb_update.bits.isValid := !mem_reg_replay && mem_cfi
   io.imem.btb_update.bits.isJump := mem_ctrl.jal || mem_ctrl.jalr
-  io.imem.btb_update.bits.isReturn := mem_ctrl.jalr && mem_reg_inst(19,15) === BitPat("b00??1")
+  io.imem.btb_update.bits.isReturn := mem_ctrl.jalr && mem_reg_inst(19,15) === BitPat("b00?01")
   io.imem.btb_update.bits.target := io.imem.req.bits.pc
   io.imem.btb_update.bits.br_pc := (if (usingCompressed) mem_reg_pc + Mux(mem_reg_rvc, UInt(0), UInt(2)) else mem_reg_pc)
   io.imem.btb_update.bits.pc := ~(~io.imem.btb_update.bits.br_pc | (coreInstBytes*fetchWidth-1))