Fix fuzz warnings and deny warnings by default in build system #1497

Job	Run time
build (--std-tests)	49s
build (--test-successful-rustc --nb-parts 2 --current-part 0)	1m 51s
build (--test-successful-rustc --nb-parts 2 --current-part 1)	1m 38s
build (--test-libcore)	51s
build (--mini-tests)	53s
success_gcc12	3s
	6m 5s

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