fix 128bits ctlz intrinsincs UB #2901

Job	Run time
duplicates	3s
spell_check	8s
build_system	25s
build (gcc-15.deb, --std-tests)	58s
build (gcc-15.deb, --extended-regex-tests)	2m 4s
build (gcc-15.deb, --extended-rand-tests)	1m 14s
build (gcc-15.deb, --projects)	2m 13s
build (gcc-15.deb, --extended-regex-example-tests)	2m 1s
build (gcc-15-without-int128.deb, --extended-regex-tests)	1m 58s
build (gcc-15-without-int128.deb, --test-libcore)	1m 1s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	1m 52s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	2m 5s
build (gcc-15-without-int128.deb, --std-tests)	1m 22s
build (gcc-15-without-int128.deb, --extended-rand-tests)	1m 45s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	1m 14s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	2m 16s
build (gcc-15-without-int128.deb, --projects)	1m 10s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	1m 3s
build (gcc-15.deb, --test-libcore)	1m 3s
success	3s
	25m 58s

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