iOS: Add IOS variable

Since commit 11da882a (Apple: Introduce separate system name for iOS,
tvOS, and watchOS, 2018-01-15, v3.14.0-rc1~14^2~1) we support setting
`CMAKE_SYSTEM_NAME` to `iOS`.  Existing iOS toolchain files already
set `IOS` as a short-hand variable, so do the same here.
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