feat: make kernel architecture-agnostic; primitive work on arm port

kernel/GNUmakefile

@@ -27,6 +27,20 @@ ifeq ($(ARCH),x86_64)
     endif
 endif
 
+# The other architectures have no in-tree cross GCC: default to LLVM, which
+# cross-compiles to every target (linking needs ld.lld; see docs/porting.md).
+ifneq ($(ARCH),x86_64)
+    ifeq ($(filter command line environment,$(origin CC)),)
+        CC := clang
+    endif
+    ifeq ($(filter command line environment,$(origin CXX)),)
+        CXX := clang++
+    endif
+    ifeq ($(filter command line environment,$(origin AR)),)
+        AR := ar
+    endif
+endif
+
 # User controllable C compiler command.
 CC  ?= cc
 
@@ -197,8 +211,10 @@ override CXXFLAGS += \
     -std=gnu++20 \
     -fno-rtti \
     -fno-exceptions \
-    -fmodules-ts \
     $(SHARED_FLAGS)
+ifneq ($(CXX_IS_CLANG),1)
+    override CXXFLAGS += -fmodules-ts
+endif
 
 # Internal linker flags that should not be changed by the user.
 override LDFLAGS += \
@@ -212,6 +228,9 @@ override LDFLAGS += \
 # Use "find" to glob all *.c, *.cpp, *.S, and *.asm files in the tree and obtain the
 # object and header dependency file names.
 override SRCFILES := $(shell cd src && find -L * -type f | LC_ALL=C sort)
+# Architecture-specific code lives in src/Arch/<arch>/; only the target's
+# directory is built. Everything else in src/ is architecture-neutral.
+override SRCFILES := $(filter-out Arch/%,$(SRCFILES)) $(filter Arch/$(ARCH)/%,$(SRCFILES))
 override CFILES := $(filter %.c,$(SRCFILES))
 override CXXFILES := $(filter %.cpp,$(SRCFILES))
 override OBJCFILES := $(filter %.mm,$(SRCFILES))
@@ -253,6 +272,11 @@ $(BUILDNO_HDR): $(filter-out $(BUILDNO_OBJ),$(OBJ)) $(PROG_SRC)
 # on a clean build (where no .d dependency exists yet).
 $(BUILDNO_OBJ): $(BUILDNO_HDR)
 
+# Compile everything without linking (used by scripts/check-arch-neutral.sh
+# to verify a port's objects before a cross linker is available).
+.PHONY: objects
+objects: $(OBJ) cc-runtime-$(ARCH)/cc-runtime.a
+
 # Link rules for building the C compiler runtime.
 cc-runtime-$(ARCH)/cc-runtime.a: GNUmakefile cc-runtime/*
 	rm -rf cc-runtime-$(ARCH)
@@ -263,10 +287,33 @@ cc-runtime-$(ARCH)/cc-runtime.a: GNUmakefile cc-runtime/*
 		CFLAGS="$(CFLAGS)" \
 		CPPFLAGS='-isystem ../freestnd-c-hdrs/$(ARCH)/include -DCC_RUNTIME_NO_FLOAT'
 
+# Clang's generic-ELF driver links cross targets through the host's GCC,
+# so a clang-built non-x86 kernel is linked by ld.lld directly.
+ifneq ($(ARCH),x86_64)
+    ifeq ($(CXX_IS_CLANG),1)
+        KERNEL_LD ?= ld.lld
+        override LD_EMULATION_aarch64 := aarch64elf
+        override LD_EMULATION_riscv64 := elf64lriscv
+        override LD_EMULATION_loongarch64 := elf64loongarch
+        override KERNEL_LDFLAGS := \
+            -m $(LD_EMULATION_$(ARCH)) \
+            --build-id=none \
+            --no-relax \
+            -static \
+            -z max-page-size=0x1000 \
+            --gc-sections \
+            -T linker-$(ARCH).ld
+    endif
+endif
+
 # Link rules for the final executable.
 bin-$(ARCH)/$(OUTPUT): GNUmakefile linker-$(ARCH).ld $(OBJ) cc-runtime-$(ARCH)/cc-runtime.a
 	mkdir -p "$$(dirname $@)"
+ifeq ($(KERNEL_LD),)
 	$(CXX) $(CXXFLAGS) $(LDFLAGS) $(OBJ) cc-runtime-$(ARCH)/cc-runtime.a -o $@
+else
+	$(KERNEL_LD) $(KERNEL_LDFLAGS) $(OBJ) cc-runtime-$(ARCH)/cc-runtime.a -o $@
+endif
 
 # Compilation rules for *.c files.
 obj-$(ARCH)/%.c.o: src/%.c GNUmakefile