mman: Add map_shadow_stack() flags

In preparation for adding arm64 GCS support make the map_shadow_stack()
SHADOW_STACK_SET_TOKEN flag generic and add _SET_MARKER. The existing
flag indicates that a token usable for stack switch should be added to
the top of the newly mapped GCS region while the new flag indicates that
a top of stack marker suitable for use by unwinders should be added
above that.

For arm64 the top of stack marker is all bits 0.

Reviewed-by: Thiago Jung Bauermann <[email protected]>
Reviewed-by: Catalin Marinas <[email protected]>
Acked-by: Yury Khrustalev <[email protected]>
Signed-off-by: Mark Brown <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

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