Dockerfile: fix kernel & initrd cross-compilation

[akerouanton]

• Automatically install gcc cross-compiler, and set ARCH and CROSS_COMPILE during kernel compilation, by comparing current arch with the supllied KERNEL_ARCH.
• Use Buildkit TARGETARCH to determine which version of crun should be downloaded, and compile the init binary for the correct architecture.

[Copilot]

Pull request overview

This PR enables cross-compilation of the kernel and initrd by automatically detecting architecture mismatches and configuring the appropriate toolchain. The changes ensure that the kernel, init binary, and crun runtime are compiled for the target architecture rather than the host architecture.

Key changes:

• Added automatic installation of cross-compiler toolchains when host and target architectures differ
• Configured kernel compilation with proper ARCH and CROSS_COMPILE environment variables for cross-compilation
• Leveraged Buildkit's TARGETARCH variable to download the correct crun binary and compile the init binary for the target platform

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[hsiangkao]

sigh, hi @akerouanton I really hope #61 could be addressed.
I don't have enough bandwidth on this since my current work doesn't allow me, but I really hope there could be a local build way without Dockerfile since the network condition in China is very poor.

[dmcgowan]

@hsiangkao can discuss in that issue, the cross compilation is not very easy right now. Some components cannot be compiled easily on Linux and some components cannot be easily compiled on Mac.