feat: add support for aarch64 timer/interrupts, port MontaukOS userspace

kernel/GNUmakefile

@@ -153,8 +153,10 @@ ifeq ($(ARCH),aarch64)
         override CXX += \
             -target aarch64-unknown-none
     endif
+    # Plain char is signed on x86; the generic kernel is written for that.
     override SHARED_FLAGS += \
-        -mgeneral-regs-only
+        -mgeneral-regs-only \
+        -fsigned-char
     override LDFLAGS += \
         -Wl,-m,aarch64elf
 endif