arch-riscv: Fix inst flags for jal and jalr

The jal and jalr share the same instruction format JumpConstructor,
which sets the IsCall and IsReturn flags by the register ID.
However, it may cause wrong instruction flags set for jal because
the section "handle the 'Jalr' instruction" misses the opcode
checking. The PR fix the issue to ensure the IsReturn can be only
set in Jalr.

Change-Id: I9ad867a389256f9253988552e6567d2b505a6901

src/arch/riscv/isa/formats/standard.isa

@@ -267,7 +267,7 @@ def template JumpConstructor {{
                 flags[IsCall] = true;
 
             // Handle "Jalr" instruction
-            if (FUNCT3 == 0x0) {
+            if (FUNCT3 == 0x0 && OPCODE5 == 0x19) {
                 // If RD is not link and RS1 is link, then pop RAS
                 if (!rd_link && rs1_link) flags[IsReturn] = true;
                 else if (rd_link) {