fix: let cppgc align to 16 bytes #6333

Job	Run time
debug x86_64-apple-darwin	29m 9s
release x86_64-apple-darwin	21m 50s
asan aarch64-apple-darwin	56m 36s
debug aarch64-apple-darwin	10m 54s
release aarch64-apple-darwin	9m 22s
debug x86_64-unknown-linux-gnu	11m 24s
release x86_64-unknown-linux-gnu	7m 33s
release x86_64-pc-windows-msvc	16m 0s
debug aarch64-unknown-linux-gnu	23m 18s
release aarch64-unknown-linux-gnu	13m 11s
publish	0s
	3h 19m 17s

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