runtime: avoid overflow in (*mheap).grow

Currently when checking if we can grow the heap into the current arena,
we do an addition which may overflow. This is particularly likely on
32-bit systems.

Avoid this situation by explicitly checking for overflow, and adding in
some comments about when overflow is possible, when it isn't, and why.

For #35954.

Change-Id: I2d4ecbb1ccbd43da55979cc721f0cd8d1757add2
Reviewed-on: https://go-review.googlesource.com/c/go/+/231337


Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>