Two memory allocator bug fixes.

- efence is not maintaining the proper heap metadata
  to make eventual memory reuse safe, so use SysFault.

- now that our heap PageSize is 8k but most hardware
  uses 4k pages, SysAlloc and SysReserve results must be
  explicitly aligned. Do that in a few more call sites and
  document this fact in malloc.h.

Fixes #7448.

LGTM=iant
R=golang-codereviews, josharian, iant
CC=dvyukov, golang-codereviews
https://golang.org/cl/71750048