Previously, the memory allocator on Plan 9 did
not free memory properly. It was only able to
free the last allocated block.

This change implements a variant of the
Kernighan & Ritchie memory allocator with
coalescing and splitting.

The most notable differences are:

- no header is prefixing the allocated blocks, since
  the size is always specified when calling sysFree,
- the free list is nil-terminated instead of circular.

Fixes #9736.
Fixes #9803.
Fixes #9952.

Change-Id: I00d533714e4144a0012f69820d31cbb0253031a3
Reviewed-on: https://go-review.googlesource.com/5524Reviewed-by: Dmitry Vyukov <dvyukov@google.com>