iOS: Fix detection of supported SDK architectures

Instead of relying on SDKSettings.plist which does not contain the
certain architectures, deduce the supported architectures by
inspecting libSystem.tbd and libSystem.dylib.

.tbd files are text files, so just parse out the archs string.
.dylib files can be fat or non-fat, so use lipo -info to extract
the architectures and parse lipo output.

Fixes: #20588