Treat f16 as a u16 in ABI #1646

Job	Run time
build (libgccjit12.so, gcc12, --no-default-features, TEST_FLAGS='-Cpanic=abort -Zpanic-abort-test...	1m 31s
build (libgccjit_without_int128.so, master-without-128bit-integers)	9m 2s
build (libgccjit.so, master)	8m 59s
success_failures	3s
	19m 35s

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