runtime: define darwin/arm64's address space as 33 bits
On iOS, the address space is not 48 bits as one might believe, since
it's arm64 hardware. In fact, all pointers are truncated to 33 bits, and
the OS only gives applications access to the range [1<<32, 2<<32).
While today this has no effect on the Go runtime, future changes which
care about address space size need this to be correct.
Updates #35112.
Change-Id: Id518a2298080f7e3d31cf7d909506a37748cc49a
Reviewed-on: https://go-review.googlesource.com/c/go/+/205758
Run-TryBot: Michael Knyszek <mknyszek@google.com> Reviewed-by: Keith Randall <khr@golang.org>