Implement runtime availability checking #3068

Job	Run time
Check MSRV	45s
Lint macOS 64bit	2m 37s
Check formatting	15s
Lint iOS 32bit	2m 22s
Lint GNUStep + exceptions	1m 42s
Lint header-translator	28s
Lint test-assembly	25s
Test macOS 12	2m 53s
Compiler UI	1m 24s
Assembly tests	46s
Verify header translator output	58s
Check framework features	0s
${{ matrix.name }}	0s
Test Compiler-RT	1m 19s
Test GNUStep with libobjc2 v1.9	41s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.0	1m 57s
Test GNUStep with libobjc2 v2.1 on nightly	51s
Test GNUStep 32bit	2m 0s
	21m 23s

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