lib/ukvmem: Make stack VMA guards size configurable and end-to-end

lib/ukvmem: Make stack VMA guards size configurable and end-to-end

Add end-to-end page guards to stack VMAs and make their size be
configurable: default 4 pages for top (to counter stack declared arrays)
and 1 page for bottom (to counter too many pop instructions).

Checkpatch-Ignore: LONG_LINE_COMMENT
Signed-off-by: Sergiu Moga <sergiu@unikraft.io>
Reviewed-by: Michalis Pappas <michalis@unikraft.io>
Approved-by: Simon Kuenzer <simon@unikraft.io>
GitHub-Closes: #1322