BUILDING.md: Add Arm64 iOS sim build instructions

Unfortunately, iOS builds cannot be used with the iOS simulator on Macs
with Apple silicon CPUs.  Even more unfortunately, universal binaries
can only have one slice for each CPU architecture, so it would not be
possible to add a dedicated Arm64 iOS simulator slice to the existing
libjpeg-turbo iOS binaries.  (It would be necessary to release a
separate package solely for the iOS simulator.)  Because the Arm Neon
SIMD extensions for libjpeg-turbo now use compiler intrinsics when
building with Xcode, it is easy to build libjpeg-turbo from source when
targeting Arm64-based Apple platforms.  Thus, for the moment, I have
chosen to document how to avoid the pothole rather than to fill it in.

BUILDING.md

@@ -429,6 +429,10 @@ iPhone 5S/iPad Mini 2/iPad Air and newer.
       [additional CMake flags] {source_directory}
     make
 
+Replace `iPhoneOS` with `iPhoneSimulator` and `-miphoneos-version-min` with
+`-miphonesimulator-version-min` to build libjpeg-turbo for the iOS simulator on
+Macs with Apple silicon CPUs.
+
 
 Building libjpeg-turbo for Android
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