target/riscv: Set vtype.vill on CPU reset

The RISC-V unprivileged specification "31.3.11. State of Vector
Extension at Reset" has a note that recommends vtype.vill be set on
reset as part of ensuring that the vector extension have a consistent
state at reset.

This change now makes QEMU consistent with Spike which sets vtype.vill
on reset.

Signed-off-by: Rob Bradford <rbradford@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Message-ID: <20240930165258.72258-1-rbradford@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>

target/riscv/cpu.c

@@ -1010,6 +1010,7 @@ static void riscv_cpu_reset_hold(Object *obj, ResetType type)
     cs->exception_index = RISCV_EXCP_NONE;
     env->load_res = -1;
     set_default_nan_mode(1, &env->fp_status);
+    env->vill = true;
 
 #ifndef CONFIG_USER_ONLY
     if (cpu->cfg.debug) {