mm: add VM_SHADOW_STACK definition for riscv

mm: add VM_SHADOW_STACK definition for riscv

VM_HIGH_ARCH_5 is used for riscv.

Reviewed-by: Zong Li <zong.li@sifive.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Acked-by: David Hildenbrand <david@redhat.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-v23-1-b55691eacf4f@rivosinc.com
[pjw@kernel.org: clarify subject; update to apply]
Signed-off-by: Paul Walmsley <pjw@kernel.org>