rts: use page sized mblocks on wasm

This patch changes mblock size to page size on wasm. It allows us to
simplify our wasi-libc fork, makes it much easier to test third party
libc allocators like emmalloc/mimalloc, as well as experimenting with
threaded RTS in wasm.

(cherry picked from commit 558353f4)
(cherry picked from commit f3ea9fb8)