os: enable TestMkdirAllWithSymlink on darwin/arm64

os: enable TestMkdirAllWithSymlink on darwin/arm64

Go 1.16 renamed the iOS port from darwin/arm64 to ios/arm64 and
darwin/arm64 was repurposed for the macOS ARM64 port (see
https://golang.org/doc/go1.16#darwin).

TestMkdirAllWithSymlink ought to run on darwin/arm64, so enable it on
that platform.

For #45696

Change-Id: I2cad6b1dfddf215e6b6cd262bbd22251f48f3d8c
Reviewed-on: https://go-review.googlesource.com/c/go/+/373359
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Emmanuel Odeke <emmanuel@orijtech.com>