riscv: create a Kconfig fragment for shadow stack and landing pad support

riscv: create a Kconfig fragment for shadow stack and landing pad support

This patch creates a Kconfig fragment for shadow stack support and
landing pad instruction support. Shadow stack support and landing pad
instruction support can be enabled by selecting
'CONFIG_RISCV_USER_CFI'. Selecting 'CONFIG_RISCV_USER_CFI' wires up
the path to enumerate CPU support.  If support exists, the kernel will
support CPU-assisted user mode CFI.

If CONFIG_RISCV_USER_CFI is selected, select 'ARCH_USES_HIGH_VMA_FLAGS',
'ARCH_HAS_USER_SHADOW_STACK' and 'DYNAMIC_SIGFRAME' for riscv.

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-v23-25-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description, Kconfig text; added CONFIG_MMU exclusion]
Signed-off-by: Paul Walmsley <pjw@kernel.org>