target/riscv: Fix henvcfg potentially containing stale bits

target/riscv: Fix henvcfg potentially containing stale bits

With the current implementation, if we had the following scenario:
- Set bit x in menvcfg
- Set bit x in henvcfg
- Clear bit x in menvcfg
then, the internal variable env->henvcfg would still contain bit x due
to both a wrong menvcfg mask used in write_henvcfg() as well as a
missing update of henvcfg upon menvcfg update.
This can lead to some wrong interpretation of the context. In order to
update henvcfg upon menvcfg writing, call write_henvcfg() after writing
menvcfg. Clearing henvcfg upon writing the new value is also needed in
write_henvcfg() as well as clearing henvcfg upper part when writing it
with write_henvcfgh().

Signed-off-by: Clément Léger <cleger@rivosinc.com>
Reviewed-by: Daniel Henrique Barboza <dbarboza@ventanamicro.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-ID: <20250110125441.3208676-2-cleger@rivosinc.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>