vlib: mmap process stacks

vlib: mmap process stacks

Instead of allocating stack from the main heap, this patch mmaps stack
memory together with guard page.

This aproach reduces main heap usage, and stack memory is prefaulted
on demand, so bigger process stacks will have zero impact on memory
usage as long as stack memory is not needed for real.

In addition, it fixes issue with systems which have bigger default page
size (observed with 65536).

Type: improvement
Change-Id: I593365c603d4702e428967d80fd425fdee2c4a21
Signed-off-by: Damjan Marion <damarion@cisco.com>