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 <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Emmanuel Odeke <bd234ba4276433f0e5fc7a8fa2d18274fa711567@orijtech.com>

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