In the kernel source directory, run
make O=/tmp/linux/arm64 ARCH=arm64 CROSS_COMPILE=aarch64-linux-gnu- -j60 defconfig all
Then create an Alpine aarch64 image following https://hackmd.io/@starnight/Run_Alpine_on_QEMU_aarch64_Virtual_Machine
wget https://dl-cdn.alpinelinux.org/alpine/latest-stable/releases/aarch64/netboot/vmlinuz-lts https://dl-cdn.alpinelinux.org/alpine/latest-stable/releases/aarch64/netboot/config-lts