Consider switching `*-uefi-*` targets to `uefi` instead of `windows` for LLVM 21+

[alexrp]

There is ongoing work to make UEFI a first-class citizen in LLVM. The backend work in particular is what's important for us here.

Once these efforts progress far enough (maybe LLVM 21?), we should consider switching over to uefi when lowering to LLVM as our use of windows for this purpose is a brittle hack that could break with any given LLVM upgrade.

zig/src/codegen/llvm.zig

Line 186 in ea527f7

.windows, .uefi => "windows",

[alexrp]

There's still ongoing work on the LLVM side that won't make it into 20, so pushing this to 0.16.0.