cmake: Simplify the iOS.cmake toolchain

- Remove a comment that was clearly copy pasted from Android.cmake and
  isn't relevant.
- Remove the toolchain's sensitivity to the environment. It's less
  error prone to just allow users to set CMAKE_OSX_SYSROOT if they
  want to use a custom SDK.
- Stop explicitly setting -mios-version-min to the default value. It
  just adds needless complexity.

This makes building the native tablegen work for me even when SDKROOT
is set in the environment (or passed in as -DCMAKE_OSX_SYSROOT).

llvm-svn: 260763