Library Forwarding: Make building on weak cross-compiling setups easier

[neobrain]

ARM64 Ubuntu provides both an x86 cross-compiler and corresponding glibc/stdlibc++ libraries, which makes cross-compiling FEX's guest library wrappers very easy. Sadly most other distributions only ship the compiler without any libraries/headers at all. In such environments, it can be easier to compile only the host-side libraries while offloading builds of the guest libraries to another machine / VM.

This PR adds two main features to make such fallback setups possible:

• Guest wrappers may now be built separately from the main project by running cmake ../ThunkLibs/CMakeLists.txt -DX86_32_TOOLCHAIN_FILE=`realpath ../toolchain_x86_32.cmake` -DX86_64_TOOLCHAIN_FILE=`realpath ../toolchain_x86_64.cmake` -DX86_DEV_ROOTFS=/ from a dedicated build folder.
• The new X86_DEV_ROOTFS CMake setting can be used to provide a development x86 rootfs, from which standard headers will be read. (Specifically, Ubuntu requires these packages: libstdc++-12-dev-amd64-cross libstdc++-12-dev-i386-cross libstdc++-12-dev libc6-dev-amd64-cross libc6-dev-i386-cross linux-libc-dev-amd64-cross linux-libc-dev-i386-cross)

[alyssarosenzweig]

In the case where we have a working x86 cross compiler when building on arm64, and can procure a x86 rootfs but without the proper multilib integration, does X86_DEV_ROOTFS provide the necessary glue to build thunks without needing a separate machine/VM? If so, I think that sounds like a decent approach for Fedora. It would be great to get some documentation (either in-tree or contributed to wiki) about how to use these new options to build fex on such weak setups.

[neobrain]

In the case where we have a working x86 cross compiler when building on arm64, and can procure a x86 rootfs but without the proper multilib integration, does X86_DEV_ROOTFS provide the necessary glue to build thunks without needing a separate machine/VM? If so, I think that sounds like a decent approach for Fedora. It would be great to get some documentation (either in-tree or contributed to wiki) about how to use these new options to build fex on such weak setups.

It might work without a VM, but I haven't tested it. You'd have to add the dev rootfs paths to the CMake toolchain files in our project root so that the CMake targets in GuestLibs/CMakeLists.txt can also find the headers/libraries. Definitely interested to see someone try that out!

[neobrain]

Realized we can achieve the same effect by using --sysroot and re-adding the global /usr/include directory for library headers, which is much cleaner than adding hardcoded include directories manually.

[neobrain]

Something's off about this. I'm getting weird RPATH errors without ../, but with ../ it's clearly putting the data in the wrong folder.