Implement runtime availability checking #3077

Job	Run time
Check MSRV	52s
Check formatting	23s
Lint macOS 64bit	2m 40s
Lint iOS 32bit	2m 11s
Lint GNUStep + exceptions	1m 24s
Lint header-translator	29s
Lint test-assembly	24s
Test macOS 12	2m 45s
Assembly tests	48s
Verify header translator output	54s
Check framework features	0s
${{ matrix.name }}	0s
Test Compiler-RT	26s
Test GNUStep with libobjc2 v1.9	1m 4s
Compiler UI	1m 46s
Test GNUStep with libobjc2 v2.0	1m 18s
Test on iOS simulator w. dinghy	0s
Test GNUStep with libobjc2 v2.1 on nightly	56s
Test GNUStep 32bit	1m 26s
	19m 46s

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