target/riscv: Fix VSTIP bit in sstc extension.

target/riscv: Fix VSTIP bit in sstc extension.

VSTIP is only writable when both [mh]envcfg.STCE is enabled, or it will
revert it's defined behavior as if sstc extension is not implemented.

Signed-off-by: Jim Shu <jim.shu@sifive.com>
Acked-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250519143518.11086-4-jim.shu@sifive.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>