Bug 1701620 part 4 - Fix memory support for WASI. r=jandem,sfink

WASI lacks of support many memory stuff like mmap, memory protections and etc, but
it has malloc so we can use it instead. Also, here we are stubbing out all
uses of the missing WASI memory functionality.

Differential Revision: https://phabricator.services.mozilla.com/D110075