target/riscv: preserve RV32 henvcfgh on henvcfg writes

On RV32, STCE/ADUE/PBMTE/DTE are implemented in henvcfgh. A write to
henvcfg should therefore only update the low 32 bits of env->henvcfg.

The current write_henvcfg() path overwrites env->henvcfg with the
low-half value and clears any bits previously written via henvcfgh.

Preserve the upper 32 bits on RV32 henvcfg writes and keep the existing
RV64 behaviour unchanged.

Signed-off-by: Bruno Sa <bruno.vilaca.sa@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20260409155344.2849233-2-bruno.vilaca.sa@gmail.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>