prctl: arch-agnostic prctl for shadow stack

Three architectures (x86, aarch64, riscv) have announced support for
shadow stacks with fairly similar functionality.  While x86 is using
arch_prctl() to control the functionality neither arm64 nor riscv uses
that interface so this patch adds arch-agnostic prctl() support to
get and set status of shadow stacks and lock the current configuation to
prevent further changes, with support for turning on and off individual
subfeatures so applications can limit their exposure to features that
they do not need.  The features are:

  - PR_SHADOW_STACK_ENABLE: Tracking and enforcement of shadow stacks,
    including allocation of a shadow stack if one is not already
    allocated.
  - PR_SHADOW_STACK_WRITE: Writes to specific addresses in the shadow
    stack.
  - PR_SHADOW_STACK_PUSH: Push additional values onto the shadow stack.

These features are expected to be inherited by new threads and cleared
on exec(), unknown features should be rejected for enable but accepted
for locking (in order to allow for future proofing).

This is based on a patch originally written by Deepak Gupta but modified
fairly heavily, support for indirect landing pads is removed, additional
modes added and the locking interface reworked.  The set status prctl()
is also reworked to just set flags, if setting/reading the shadow stack
pointer is required this could be a separate prctl.

Reviewed-by: Thiago Jung Bauermann <thiago.bauermann@linaro.org>
Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Yury Khrustalev <yury.khrustalev@arm.com>
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Deepak Gupta <debug@rivosinc.com>
Link: https://lore.kernel.org/r/20241001-arm64-gcs-v13-4-222b78d87eee@kernel.org


Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>