oslib-posix: add helpers for stack alloc and free

the allocated stack will be adjusted to the minimum supported stack size
by the OS and rounded up to be a multiple of the system pagesize.
Additionally an architecture dependent guard page is added to the stack
to catch stack overflows.

Signed-off-by: Peter Lieven <pl@kamp.de>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
