On most systems, a pointer is the worst case alignment, so adding
a pointer field at the end of a struct guarantees there will be no
padding added after that field (to satisfy overall struct alignment
due to some more-aligned field also present).

In the runtime, the map implementation needs a quick way to
get to the overflow pointer, which is last in the bucket struct,
so it uses size - sizeof(pointer) as the offset.

NaCl/amd64p32 is the exception, as always.
The worst case alignment is 64 bits but pointers are 32 bits.
There's a long history that is not worth going into, but when
we moved the overflow pointer to the end of the struct,
we didn't get the padding computation right.
The compiler computed the regular struct size and then
on amd64p32 added another 32-bit field.
And the runtime assumed it could step back two 32-bit fields
(one 64-bit register size) to get to the overflow pointer.
But in fact if the struct needed 64-bit alignment, the computation
of the regular struct size would have added a 32-bit pad already,
and then the code unconditionally added a second 32-bit pad.
This placed the overflow pointer three words from the end, not two.
The last two were padding, and since the runtime was consistent
about using the second-to-last word as the overflow pointer,
no harm done in the sense of overwriting useful memory.
But writing the overflow pointer to a non-pointer word of memory
means that the GC can't see the overflow blocks, so it will
collect them prematurely. Then bad things happen.

Correct all this in a few steps:

1. Add an explicit check at the end of the bucket layout in the
compiler that the overflow field is last in the struct, never
followed by padding.

2. When padding is needed on nacl (not always, just when needed),
insert it before the overflow pointer, to preserve the "last in the struct"
property.

3. Let the compiler have the final word on the width of the struct,
by inserting an explicit padding field instead of overwriting the
results of the width computation it does.

4. For the same reason (tell the truth to the compiler), set the type
of the overflow field when we're trying to pretend its not a pointer
(in this case the runtime maintains a list of the overflow blocks
elsewhere).

5. Make the runtime use "last in the struct" as its location algorithm.

This fixes TestTraceStress on nacl/amd64p32.
The 'bad map state' and 'invalid free list' failures no longer occur.

Fixes #11838.

Change-Id: If918887f8f252d988db0a35159944d2b36512f92
Reviewed-on: https://go-review.googlesource.com/12971Reviewed-by: Keith Randall <khr@golang.org>
Reviewed-by: Austin Clements <austin@google.com>