arch/riscv: compile vdso with landing pad and shadow stack note

arch/riscv: compile vdso with landing pad and shadow stack note

User mode tasks compiled with Zicfilp may call indirectly into the
vdso (like hwprobe indirect calls). Add support for compiling landing
pads into the vdso. Landing pad instructions in the vdso will be
no-ops for tasks which have not enabled landing pads. Furthermore, add
support for the C sources of the vdso to be compiled with shadow stack
and landing pads enabled as well.

Landing pad and shadow stack instructions are emitted only when the
VDSO_CFI cflags option is defined during compile.

Signed-off-by: Jim Shu <jim.shu@sifive.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-v23-23-b55691eacf4f@rivosinc.com
[pjw@kernel.org: cleaned up patch description, issues reported by checkpatch]
Signed-off-by: Paul Walmsley <pjw@kernel.org>