runtime: fix malloc page alignment + efence
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