target/riscv/csr.c: Fix an access to VXSAT

target/riscv/csr.c: Fix an access to VXSAT

The register VXSAT should be RW only to the first bit.
The remaining bits should be 0.

The RISC-V Instruction Set Manual Volume I: Unprivileged Architecture

The vxsat CSR has a single read-write least-significant bit (vxsat[0])
that indicates if a fixed-point instruction has had to saturate an output
value to fit into a destination format. Bits vxsat[XLEN-1:1]
should be written as zeros.

Signed-off-by: Evgenii Prokopiev <evgenii.prokopiev@syntacore.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20241002084436.89347-1-evgenii.prokopiev@syntacore.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>