Fix GNUStep NSMutableData new #2124

Job	Run time
Check MSRV	1m 29s
Check formatting	15s
Lint macOS 64bit	4m 52s
Lint iOS 32bit	1m 7s
Lint GNUStep + exceptions	2m 21s
Lint header-translator	37s
Lint test-assembly	43s
Test macOS 12	16m 20s
Verify header translator output	1m 6s
Assembly tests	5m 4s
Check icrate features	17m 22s
Test macOS 11	10m 4s
Build macOS AArch64	13m 18s
Test macOS old SDK	16m 46s
Test macOS nightly	11m 44s
Build macOS 32bit	15m 47s
Build iOS simulator ARM64	3m 26s
Build iOS ARM64	5m 20s
Build iOS ARMv7	5m 48s
Build iOS ARMv7s	9m 13s
Build iOS 32bit x86	6m 27s
Test Compiler-RT	43s
Test on iOS simulator w. dinghy	9m 53s
Compiler UI	1m 12s
Test GNUStep with libobjc2 v1.9	8m 59s
Test GNUStep with libobjc2 v2.0	7m 18s
Test GNUStep with libobjc2 v2.1 on nightly	5m 46s
Test GNUStep 32bit	5m 10s
	3h 8m 10s

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