Treat f16 as a u16 in ABI #3400

Job	Run time
spell_check	12s
build_system	24s
duplicates	4s
build (gcc-15.deb, --extended-regex-tests)	11m 29s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	14m 27s
build (gcc-15.deb, --test-libcore)	14m 35s
build (gcc-15-without-int128.deb, --test-libcore)	13m 44s
build (gcc-15.deb, --projects)	13m 7s
build (gcc-15.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	14m 30s
build (gcc-15-without-int128.deb, --extended-rand-tests)	8m 58s
build (gcc-15-without-int128.deb, --extended-regex-example-tests)	7m 56s
build (gcc-15.deb, --extended-regex-example-tests)	6m 27s
build (gcc-15.deb, --extended-rand-tests)	8m 44s
build (gcc-15-without-int128.deb, --std-tests)	5m 31s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 1)	14m 47s
build (gcc-15.deb, --std-tests)	6m 5s
build (gcc-15-without-int128.deb, --test-successful-rustc --nb-parts 2 --current-part 0)	13m 53s
build (gcc-15-without-int128.deb, --projects)	15m 29s
build (gcc-15-without-int128.deb, --extended-regex-tests)	11m 2s
success	3s
	3h 1m 27s

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