On Linux, both APIs uses {madvise} (and {SetPermissions} additionally
does an {mprotect}), on windows they use different APIs. It turns out that
{SetPermissions} with {kNoAccess} uses {VirtualFree} with
{MEM_DECOMMIT}, so the pages will only be "reserved" afterwards.
{DiscardSystemPages} on the other hand uses the {DiscardVirtualMemory}
which tells the OS that the content of that memory can be discarded,
but the pages stay "committed".
Thus {SetPermissions} with {kNoAccess} frees significantly more memory
than using {DiscardSystemPages}.

R=mstarzinger@chromium.org

Bug: v8:8217
Change-Id: I3555462a9c692f15ecb3eca7834eb181617d9ef9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1730992Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Cr-Commit-Position: refs/heads/master@{#63029}