Integrate libffi in the build system

[alexrp]

This will be needed for the interpreter to call external functions.

I don't want to take a dependency on the system libffi since that majorly complicates our (currently) simple build process. That said, it should still be possible to use the system libffi for distro maintainers (https://github.com/ziglang/zig/pull/18778). Also, as with Aro, it should be possible to disable libffi support since I expect very few Graf library users will care about the interpreter.

This will likely involve forking libffi, adding a build.zig, and then adding that fork as a lazy build.zig.zon dependency (but note #16).

[alexrp]

Started the work here: https://github.com/alexrp/libffi/commits/master

The script itself mostly works, but I ran into a handful of issues:

• https://github.com/ziglang/zig/issues/10411 (affects arm-linux-gnueabi)
• https://github.com/ziglang/zig/issues/19107 (affects riscv64-linux-gnu)
• https://github.com/ziglang/zig/issues/20361 (affects all Apple targets)
• https://github.com/ziglang/fetch-them-macos-headers/issues/28 (affects aarch64-macos-none with pauth CPU feature; not strictly required)
• https://github.com/llvm/llvm-project/issues/58377 (affects all MIPS targets)
• No GNU as-compatible assembly files for aarch64-windows-gnu (libffi issue)

Also, I raised the question of upstreaming the build script: https://github.com/libffi/libffi/issues/842

[alexrp]

For now, we'll just have to maintain some logic to disable libffi support on known-broken targets.