riscv: fix mstatus/sstatus helpers and field mask

[elliott10]

riscv: complete mstatus and sstatus status helpers

Align the mstatus and sstatus helpers with the privileged spec.

• add missing base-field helpers for UBE, VS, UXL and SXL/SBE/MBE where applicable
• model XS and SD as read-only summary fields instead of writable fields
• keep set_uie/set_upie as compatibility helpers for legacy N-extension users
• fix the CSR masks to preserve all legal status bits
• update tests to cover the new helpers and read-only field behavior

[romancardenas]

I will need some time to review this PR. Can you share with me the link to the ISA specification you used for these modifications? I usually use this: https://docs.riscv.org/reference/isa/_attachments/riscv-privileged.pdf

There is a new release (from February) and looks like there is a few more fields

[elliott10]

I will need some time to review this PR. Can you share with me the link to the ISA specification you used for these modifications? I usually use this: https://docs.riscv.org/reference/isa/_attachments/riscv-privileged.pdf

There is a new release (from February) and looks like there is a few more fields

I referred to this ISA Spec: https://docs.riscv.org/reference/isa/priv/supervisor.html#sstatus

If there is anything I have missed, I am willing to keep improving it ：）

[romancardenas]

Looks good to me!! Please, address the changes in CHANGELOG.md and the minor docs formatting comment.

[romancardenas]

Sorry, I approved the PR by mistake. Address the changes before merging

[elliott10]

Sorry, I approved the PR by mistake. Address the changes before merging

Ok, I've made the changes.
And, I have merged all the modifications into one commit.

[romancardenas]

Thanks!