following https://mesonbuild.com/Cross-compilation.html
- host_machine is our XBPS_TARGET_MACHINE
- build_machine is our XBPS_MACHINE, and meson can find out the details
on it's own
- also don't include -musl in the CPU because meson doesn't recognize it
and projects like Mesa (LibGL) don't enable optimizations for it
- cpu_family and cpu are different, they need to be set properly:
- armv* is arm
- mips* is mips
- i686 is x86
- x86_64 is x86_64
- aarch64 is aarch64
