target/riscv: Set vtype.vill on CPU reset

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>
(cherry picked from commit f8c1f36a2e3dab4935e7c5690e578ac71765766b)
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>