testing/systemd-efistub: new aport

As mentioned in #14679 (closed), the gummiboot EFI stub relies on CONFIG_EFI_HANDOVER_PROTOCOL, which has been depreciated in the Linux kernel. Since gummiboot has been merged into SystemD-boot, the gummiboot EFI stub will not be updated to fix this. @WhyNotHugo created a testing package containing systemd-boot, systemd-efistub and ukify in !61369 (closed), but this was closed. After finding my laptop unable to boot using the gummiboot-efistub package, I have updated his pull request to work with SystemD 257. Building the efistib on Musl is simple and only requires patching wchar_t to uint16_t, and ukify is a pure python package. However, unlike !61369 (closed), I have chosen not to package systemd-boot. This is because (to my understanding) systemd-boot requires the bootctl SystemD CLI tool to be useful, which is integrated into SystemD and so much more complicated to package.

I have tested this on two x86_64 laptops, and using VMs for x86, aarch64 and armv7.