fix 128bits ctlz intrinsincs UB #2884

Job	Run time
duplicates	5s
spell_check	11s
build (gcc-15.deb, --test-libcore)	15m 55s
build_system	45s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 53s
build (gcc-15.deb, --extended-rand-tests)	9m 46s
build (gcc-15.deb, --std-tests)	6m 53s
build (gcc-15.deb, --extended-regex-example-tests)	6m 33s
build (gcc-15.deb, --extended-regex-tests)	12m 56s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	13m 13s
build (gcc-15-without-int128.deb, --std-tests)	2m 42s
build (gcc-15.deb, --projects)	11m 1s
build (gcc-15-without-int128.deb, --extended-rand-tests)	2m 48s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	3m 9s
build (gcc-15-without-int128.deb, --projects)	2m 47s
build (gcc-15-without-int128.deb, --test-libcore)	2m 55s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	2m 45s
build (gcc-15-without-int128.deb, --extended-regex-tests)	2m 58s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	2m 2s
success	2s
	1h 53m 19s

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