CMakeIOSInstallCombined: Enforce disjoint architecture sets

Starting with Xcode 12 the arm64 architecture is supported
as an iOS device as well as simulator architecture.

But the fat macho file format does not distinguish by SDK,
only by architecture. That makes lipo (rightfully) complain
that it cannot add both architectures to a single file.

To work around we make sure that both SDKs are built for a
disjoint set of architectures. If an architecture is present
for both SDKs we prefer the currently configured one.

The log output has been extended to reflect that:

```
[iOS combined] Architectures (iphoneos): arm64 arm64e armv7 armv7s
[iOS combined] Architectures (iphonesimulator): arm64 arm64e i386 x86_64
[iOS combined] Architectures (iphonesimulator) after pruning: i386 x86_64
```
58 jobs for !5111 with ios-combined-prune-archs in 5 minutes and 5 seconds (queued for 2 seconds)
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Status Name Job ID Coverage
  Build
manual build:centos6-release #5124421
build docker linux manual
passed build:debian10-iwyu #5124418
build docker linux

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manual build:fedora31-ninja #5124422
build docker linux linux-3.17 manual
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manual build:macos-makefiles #5124424
cmake macos shell xcode-11.5 nonconcurrent manual
manual build:macos-ninja #5124423
cmake macos shell xcode-11.5 nonconcurrent manual
manual build:windows-vs2019-x64-ninja #5124425
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build docker linux linux-3.17
created test:macos-makefiles #5124428
cmake macos shell xcode-11.5 nonconcurrent
created test:macos-ninja #5124427
cmake macos shell xcode-11.5 nonconcurrent
created test:windows-vs2019-x64-ninja #5124429
cmake windows shell vs2019 msvc-19.25 nonconcurrent
 
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cuda-rt docker linux
created test:fedora31-makefiles #5124430
build docker linux linux-3.17
created test:fedora31-ninja-multi #5124432
build docker linux linux-3.17
created test:macos-xcode #5124433
cmake macos shell xcode-11.5 concurrent
created test:windows-vs2019-x64 #5124434
cmake windows shell vs2019 msvc-19.25 concurrent
 
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