runtime: align allocations harder in GODEBUG=sbrk=1 mode

Currently, GODEBUG=sbrk=1 mode aligns allocations by their type's
alignment. You would think this would be the right thing to do, but
because 64-bit fields are only 4-byte aligned right now (see #599),
this can cause a 64-bit field of an allocated object to be 4-byte
aligned, but not 8-byte aligned. If there is an atomic access to that
unaligned 64-bit field, it will crash.

This doesn't happen in normal allocation mode because the
size-segregated allocation and the current size classes will cause any
types larger than 8 bytes to be 8 byte aligned.

We fix this by making sbrk=1 mode use alignment based on the type's
size rather than its declared alignment. This matches how the tiny
allocator aligns allocations.

This was tested with

  GOARCH=386 GODEBUG=sbrk=1 go test sync/atomic

This crashes with an unaligned access before this change, and passes
with this change.

This should be reverted when/if we fix #599.

Fixes #33159.

Change-Id: Ifc52c72c6b99c5d370476685271baa43ad907565
Reviewed-on: https://go-review.googlesource.com/c/go/+/186919


Run-TryBot: Austin Clements <austin@google.com>
Reviewed-by: Keith Randall <khr@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>