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 (ziglang/zig#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:

• CPU features are not passed to clang when assembling ziglang/zig#10411 (affects arm-linux-gnueabi)
• Unable to cross-compile riscv64 Hello world program in c under wsl2 ziglang/zig#19107 (affects riscv64-linux-gnu)
• Bundled macOS headers cannot be found using #include in assembly files ziglang/zig#20361 (affects all Apple targets)
• ptrauth.h is missing ziglang/fetch-them-macos-headers#28 (affects aarch64-macos-none with pauth CPU feature; not strictly required)
• ld.lld: error: relocation R_MIPS_32 cannot be used against local symbol; recompile with -fPIC llvm/llvm-project#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: libffi/libffi#842

[alexrp]

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