riscv/ptrace: expose riscv CFI status and state via ptrace and in core files

riscv/ptrace: expose riscv CFI status and state via ptrace and in core files

Expose a new register type NT_RISCV_USER_CFI for risc-v CFI status and
state. Intentionally, both landing pad and shadow stack status and
state are rolled into the CFI state. Creating two different
NT_RISCV_USER_XXX would not be useful and would waste a note
type. Enabling, disabling and locking the CFI feature is not allowed
via ptrace set interface. However, setting 'elp' state or setting
shadow stack pointer are allowed via the ptrace set interface.  It is
expected that 'gdb' might need to fixup 'elp' state or 'shadow stack'
pointer.

Signed-off-by: Deepak Gupta <debug@rivosinc.com>
Tested-by: Andreas Korb <andreas.korb@aisec.fraunhofer.de> # QEMU, custom CVA6
Tested-by: Valentin Haudiquet <valentin.haudiquet@canonical.com>
Link: https://patch.msgid.link/20251112-v5_user_cfi_series-v23-19-b55691eacf4f@rivosinc.com
[pjw@kernel.org: updated to apply; cleaned patch description and comments; addressed checkpatch issues]
Signed-off-by: Paul Walmsley <pjw@kernel.org>