Implement runtime availability checking #3069

Job	Run time
Check MSRV	58s
Lint macOS 64bit	2m 47s
Check formatting	8s
Lint iOS 32bit	2m 12s
Lint GNUStep + exceptions	1m 30s
Lint header-translator	26s
Lint test-assembly	28s
Test macOS 12	3m 7s
Verify header translator output	59s
Assembly tests	50s
Check framework features	0s
${{ matrix.name }}	0s
Test Compiler-RT	22s
Test GNUStep with libobjc2 v1.9	1m 8s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.0	1m 6s
Compiler UI	1m 48s
Test GNUStep with libobjc2 v2.1 on nightly	53s
Test GNUStep 32bit	1m 20s
	20m 2s

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