sim: Fix RISC-V multiply instructions on simulator

Because of recent 'Zmmul' support, the simulator is broken.  This is caused
by instruction classification changes:

[Before]
-   INSN_CLASS_M     : multiply / divide
[After Zmmul]
-   INSN_CLASS_M     : divide
-   INSN_CLASS_ZMMUL : multiply

The simulator checks the instruction class to execute an instruction:

-   INSN_CLASS_I  : 'I'
-   INSN_CLASS_M  : 'M' (multiply / divide)
-   INSN_CLASS_A  : 'A'

'Zmmul' moved multiply instructions to INSN_CLASS_ZMMUL and that instruction
class is not handled by the simulator.

This commit handles INSN_CLASS_ZMMUL for all 'M' instructions and adds a
testcase to test all RV32M instructions run without any faults.

sim/ChangeLog:

	* riscv/sim-main.c (execute_one): Add INSN_CLASS_ZMMUL
	to run multiply instructions correctly.
	* testsuite/riscv/m-ext.s: New test.

sim/riscv/sim-main.c

@@ -936,6 +936,7 @@ execute_one (SIM_CPU *cpu, unsigned_word iw, const struct riscv_opcode *op)
     case INSN_CLASS_I:
       return execute_i (cpu, iw, op);
     case INSN_CLASS_M:
+    case INSN_CLASS_ZMMUL:
       return execute_m (cpu, iw, op);
     default:
       TRACE_INSN (cpu, "UNHANDLED EXTENSION: %d", op->insn_class);

sim/testsuite/riscv/m-ext.s

@@ -0,0 +1,18 @@
+# check that the RV32M instructions run without any fault.
+# mach: riscv
+
+.include "testutils.inc"
+
+	start
+
+	.option	arch, +m
+	mul	x0, x1, x2
+	mulh	x0, x1, x2
+	mulhu	x0, x1, x2
+	mulhsu	x0, x1, x2
+	div	x0, x1, x2
+	divu	x0, x1, x2
+	rem	x0, x1, x2
+	remu	x0, x1, x2
+
+	pass