Implement runtime availability checking #3072

Job	Run time
Check formatting	9s
Lint macOS 64bit	2m 44s
Check MSRV	46s
Lint iOS 32bit	2m 12s
Lint GNUStep + exceptions	1m 27s
Lint header-translator	31s
Lint test-assembly	29s
Test macOS 12	3m 53s
Assembly tests	55s
Verify header translator output	1m 6s
Check framework features	0s
${{ matrix.name }}	0s
Test Compiler-RT	31s
Test GNUStep with libobjc2 v1.9	1m 28s
Compiler UI	1m 34s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.0	1m 15s
Test GNUStep with libobjc2 v2.1 on nightly	1m 7s
Test GNUStep 32bit	1m 10s
	21m 17s

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