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 Job ID Name Coverage
  Build
manual build:centos6-release #5124421
linux docker build manual
passed build:debian10-iwyu #5124418
linux docker build

00:04:18

manual build:fedora31-ninja #5124422
linux-3.17 linux docker build manual
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manual build:macos-makefiles #5124424
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manual build:macos-ninja #5124423
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manual build:windows-vs2019-x64-ninja #5124425
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created test:fedora31-ninja #5124426
linux-3.17 linux docker build
created test:macos-makefiles #5124428
nonconcurrent xcode-11.5 shell macos cmake
created test:macos-ninja #5124427
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created test:windows-vs2019-x64-ninja #5124429
nonconcurrent msvc-19.25 vs2019 shell windows cmake
 
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created test:fedora31-makefiles #5124430
linux-3.17 linux docker build
created test:fedora31-ninja-multi #5124432
linux-3.17 linux docker build
created test:macos-xcode #5124433
concurrent xcode-11.5 shell macos cmake
created test:windows-vs2019-x64 #5124434
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