pin the kernel to LLVM+LLD

The self-hosted linker ignores parts of linker.ld (PHDRS, /DISCARD/,
section order). The higher-half move needs the script authoritative.
Suite 27/27 on the LLD build.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

build.zig

@@ -138,6 +138,11 @@ pub fn build(b: *std.Build) void {
     });
     exe.setLinkerScript(b.path("src/kernel/arch/x86_64/linker.ld"));
     exe.entry = .{ .symbol_name = "_start" };
+    // The self-hosted linker ignores parts of the linker script (PHDRS,
+    // /DISCARD/, section order); the higher-half layout depends on the script
+    // being authoritative, so pin the kernel to LLVM + LLD.
+    exe.use_llvm = true;
+    exe.use_lld = true;
     // Physical address the bootloader loads the kernel to (identity-mapped under
     // UEFI). Overrides Zig's default image base so the linker script's layout is
     // honoured; adjust here if it collides with firmware-reserved memory.