linker: Fix __stackmem section with CONFIG_KERNEL_COHERENCE

When building whith CONFIG_KERNEL_COHERENCE enabled,
__stackmem was not being put in the user_stack section making
impossible to gen_kobject script find user capable stacks.

It is responsability of the platform to put user_stacks
section in the appropriated region.

Signed-off-by: Flavio Ceolin <flavio.ceolin@intel.com>