runtime: avoid overflow from linearAlloc

runtime: avoid overflow from linearAlloc

Currently linearAlloc manages an exclusive "end" address for the top of
its reserved space. While unlikely for a linearAlloc to be allocated
with an "end" address hitting the top of the address space, it is
possible and could lead to overflow.

Avoid overflow by chopping off the last byte from the linearAlloc if
it's bumping up against the top of the address space defensively. In
practice, this means that if 32-bit platforms map the top of the address
space and use the linearAlloc to acquire arenas, the top arena will not
be usable.

Fixes #35954.

Change-Id: I512cddcd34fd1ab15cb6ca92bbf899fc1ef22ff6
Reviewed-on: https://go-review.googlesource.com/c/go/+/231338
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>