feat: aarch64 port - fix build system obstacles, add exception handling, page tables

kernel/GNUmakefile

@@ -291,7 +291,9 @@ cc-runtime-$(ARCH)/cc-runtime.a: GNUmakefile cc-runtime/*
 # so a clang-built non-x86 kernel is linked by ld.lld directly.
 ifneq ($(ARCH),x86_64)
     ifeq ($(CXX_IS_CLANG),1)
-        KERNEL_LD ?= ld.lld
+        # Without a system lld, fall back to the one rustup ships.
+        KERNEL_LD ?= $(firstword $(shell command -v ld.lld) \
+            $(wildcard $(HOME)/.rustup/toolchains/*/lib/rustlib/*/bin/gcc-ld/ld.lld))
         override LD_EMULATION_aarch64 := aarch64elf
         override LD_EMULATION_riscv64 := elf64lriscv
         override LD_EMULATION_loongarch64 := elf64loongarch