std.Target: Remove Abi.gnuilp32.

* This has not seen meaningful development for about a decade.
* The Linux kernel port was never upstreamed.
* The glibc port was never upstreamed.
* GCC 15.1 recently deprecated support it.

It may still make sense to support an ILP32 ABI on AArch64 more broadly (which
we already have the Abi.ilp32 tag for), but, to the extent that it even existed
in any "official" sense, the *GNU* ILP32 ABI is certainly dead.

src/codegen/llvm.zig

@@ -259,7 +259,6 @@ pub fn targetTriple(allocator: Allocator, target: std.Target) ![]const u8 {
         .gnuf32 => "gnuf32",
         .gnusf => "gnusf",
         .gnux32 => "gnux32",
-        .gnuilp32 => "gnu_ilp32",
         .code16 => "code16",
         .eabi => "eabi",
         .eabihf => "eabihf",