riscv: enable kernel access to shadow stack memory via the FWFT SBI call

riscv: enable kernel access to shadow stack memory via the FWFT SBI call

The kernel has to perform shadow stack operations on the user shadow stack.
During signal delivery and sigreturn, the shadow stack token must be
created and validated respectively. Thus shadow stack access for the kernel
must be enabled.

In the future, when kernel shadow stacks are enabled, they must be
enabled as early as possible for better coverage and to prevent any
imbalance between the regular stack and the shadow stack. After
'relocate_enable_mmu' has completed, this is the earliest that it can
be enabled.

Reviewed-by: Zong Li <zong.li@sifive.com>
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-v23-22-b55691eacf4f@rivosinc.com
[pjw@kernel.org: updated to apply; cleaned up commit message]
Signed-off-by: Paul Walmsley <pjw@kernel.org>