ARM64 JIT: don't use `x18` register #27

Job	Run time
build-freebsd (freebsd, x86-64, 13.2, ubuntu-22.04)	10m 9s
build-macos (macos-11)	44s
build-ubuntu (ubuntu-20.04, gcc-11, g++-11)	58s
build-freebsd (freebsd, arm64, 13.2, ubuntu-22.04)	6m 35s
build-alpine (x86_64, latest-stable)	35s
build-macos (macos-12)	32s
build-ubuntu (ubuntu-22.04, gcc-12, g++-12)	39s
build-alpine (x86, latest-stable)	38s
build-macos (macos-13)	47s
build-alpine (aarch64, latest-stable)	3m 0s
build-windows-msbuild (x64, 2019, Visual Studio 16 2019, C:\Program Files (x86)\Microsoft Visual ...	2m 20s
build-windows-msys2 (gcc, g++)	4m 32s
build-alpine (armhf, latest-stable)	2m 40s
build-windows-msbuild (x64, 2022, Visual Studio 17 2022, C:\Program Files\Microsoft Visual Studio...	1m 33s
build-windows-msys2 (clang, clang++)	4m 25s
build-alpine (armv7, latest-stable)	2m 48s
build-windows-msbuild (Win32, 2019, Visual Studio 16 2019, C:\Program Files (x86)\Microsoft Visua...	1m 43s
build-alpine (ppc64le, latest-stable)	3m 21s
build-windows-msbuild (Win32, 2022, Visual Studio 17 2022, C:\Program Files\Microsoft Visual Stud...	3m 42s
build-alpine (riscv64, edge)	3m 43s
build-alpine (s390x, latest-stable)	4m 12s
	59m 36s

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