WIP: main/rust: enable support for x86_64-unknown-uefi

This is an attempt to enable support for building binaries for x86_64-unknown-uefi:

https://doc.rust-lang.org/nightly/rustc/platform-support/unknown-uefi.html#building-rust-for-uefi-targets

This target is used to build programs like bootloaders written in Rust.

I experimentally added this target to the existing APKBUILD, but that fails with:

member 9dae5bf9358c6b08-absvdi2.o in .../aports/main/rust/src/rustc-1.76.0-src/build/x86_64-alpine-linux-musl/stage2-std/x86_64-unknown-uefi/release/deps/libcompiler_builtins-8fad036dccce1330.rlib is not COFF

The entire output is available at:

https://paste.sr.ht/blob/fd2658d971fd6a94a0c9ab7a05e9e8e72edd3a52

The error just tells me that when trying to build, it found an rlib (intermediate artifacts in rust builds) instead of a COFF (the uefi executable format).

I have no idea if my approach is entirely wrong, if this is an issue in how alpine packages things downstream, or this is a bug that I should ask about upstream.

I'm publishing this draft non-working MR in hopes of feedback on what to try next.