jit: Extract a64 assembler. #401

Job	Run time
host=ubuntu-22.04 cmd=cli target=x86_64-linux-gnu mode=release	1m 5s
host=ubuntu-22.04 cmd=cli target=x86_64-selinux-gnu mode=release	52s
host=macos-12 cmd=cli target=x86_64-macos-none mode=release	2m 15s
host=macos-12 cmd=cli target=aarch64-macos-none mode=release	2m 8s
host=windows-2022 cmd=cli target=x86_64-windows-gnu mode=release	1m 58s
host=windows-2022 cmd=lib target=x86_64-windows-gnu mode=release	1m 57s
host=ubuntu-22.04 cmd=web-lib target=wasm32-freestanding mode=release	1m 1s
host=ubuntu-22.04 cmd=cli target=wasm32-wasi mode=release	41s
host=ubuntu-22.04 cmd=lib target=x86_64-linux-gnu mode=release	1m 17s
host=macos-12 cmd=lib target=x86_64-macos-none mode=release	1m 35s
host=macos-12 cmd=lib target=aarch64-macos-none mode=release	1m 30s
Release	0s
	16m 19s

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