os: simplify ios checks in tests

os: simplify ios checks in tests

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).

Change tests to only use GOOS=ios to detect special cases for iOS and
stop treating darwin/arm64 as iOS.

Change-Id: I6a19f16fa9ec159ab1386aeb8fe912585e457171
Reviewed-on: https://go-review.googlesource.com/c/go/+/374399
Trust: Tobias Klauser <tobias.klauser@gmail.com>
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>