riscv/mm: teach pte_mkwrite to manufacture shadow stack PTEs

riscv/mm: teach pte_mkwrite to manufacture shadow stack PTEs

pte_mkwrite() creates PTEs with WRITE encodings for the underlying
architecture.  The underlying architecture can have two types of
writeable mappings: one that can be written using regular store
instructions, and another one that can only be written using
specialized store instructions (like shadow stack stores).
pte_mkwrite can select write PTE encoding based on VMA range (i.e.
VM_SHADOW_STACK)

Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
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-v23-8-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description]
Signed-off-by: Paul Walmsley <pjw@kernel.org>