Fix aarch64 lld-link resolution in Makefile.starkernel

The aarch64 loader link step hardcoded the unversioned "lld-link", which
isn't on PATH by default on this Debian/Ubuntu setup (the package only
installs lld-link-18 under /usr/bin; unversioned lld-link lives under
/usr/lib/llvm-18/bin). CI worked around this with an explicit PATH prefix
in the workflow; a local build without that PATH override failed. Now
auto-detects whichever name resolves, falling back to the versioned name.
Verified: aarch64 builds clean with the default PATH.

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

Makefile.starkernel

@@ -139,7 +139,10 @@ else ifeq ($(ARCH),aarch64)
     endif
     # aarch64 loader uses clang + lld-link (COFF target — GCC cannot produce aarch64 PE)
     LOADER_CC            := clang-18
-    LOADER_LD            := lld-link
+    # Debian/Ubuntu package only installs the versioned lld-link-18 on PATH by
+    # default (unversioned lld-link lives under /usr/lib/llvm-18/bin, which
+    # isn't on PATH unless explicitly prepended) — detect whichever resolves.
+    LOADER_LD            := $(shell which lld-link 2>/dev/null || which lld-link-18 2>/dev/null || echo lld-link)
     ARCH_CFLAGS          := -march=armv8-a -mcpu=cortex-a72 -DARCH_AARCH64 -mno-outline-atomics
     LOADER_LINKER_SCRIPT := linker/starkernel-loader-aarch64.ld
     KERNEL_LINKER_SCRIPT := linker/starkernel-kernel-aarch64.ld
@@ -609,7 +612,7 @@ $(KERNEL_OBJ_DIR)/%.o: $(KERNEL_SRC)/%.S | $(KERNEL_OBJ_DIR)
 $(LOADER_EFI): $(LOADER_OBJS) | $(BUILD_DIR)
 ifeq ($(ARCH),aarch64)
 	@echo "LLD-LINK (loader PE) $@"
-	@lld-link /subsystem:EFI_APPLICATION /entry:efi_main /nodefaultlib \
+	@$(LOADER_LD) /subsystem:EFI_APPLICATION /entry:efi_main /nodefaultlib \
 	    /align:4096 /base:0 /out:$@ $(LOADER_OBJS)
 	@echo "Loader built: $@"
 else ifeq ($(ARCH),riscv64)