runtime: use 4 MiB heap arenas on iOS

iOS arm64 is a 64-bit platform but with a strictly 32-bit address space
(technically 33 bits, but the bottom half is unavailable to the
application). Since address space is limited, use 4 MiB arenas instead
of 64 MiB arenas. No changes are needed to the arena index because it's
still relatively small; this change just brings iOS more in line with
32-bit platforms.

Change-Id: I484e2d273d896fd0a57cd5c25012df0aef160290
Reviewed-on: https://go-review.googlesource.com/c/go/+/270538


Trust: Michael Knyszek <mknyszek@google.com>
Trust: Emmanuel Odeke <emmanuel@orijtech.com>
Trust: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Michael Knyszek <mknyszek@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>