riscv32-nemu: add integer library functions to pass linking

* they are called in __udivmoddi4(), which is used with uint64_t
  division in klib/src/stdio.c

am/arch/platform/nemu.mk

@@ -18,7 +18,7 @@ NEMU_ARGS = -b $(MAINARGS) -l $(shell dirname $(BINARY))/nemu-log.txt $(BINARY).
 
 image:
 	@echo + LD "->" $(BINARY_REL).elf
-	@$(LD) $(LDFLAGS) --gc-sections -T $(LD_SCRIPT) -e _start -o $(BINARY).elf $(LINK_FILES)
+	@$(LD) $(LDFLAGS) --gc-sections -T $(LD_SCRIPT) -e _start -o $(BINARY).elf --start-group $(LINK_FILES) --end-group
 	@$(OBJDUMP) -d $(BINARY).elf > $(BINARY).txt
 	@echo + OBJCOPY "->" $(BINARY_REL).bin
 	@$(OBJCOPY) -S --set-section-flags .bss=alloc,contents -O binary $(BINARY).elf $(BINARY).bin

am/arch/riscv32-nemu.mk

@@ -1,2 +1,4 @@
 include $(AM_HOME)/am/arch/isa/riscv32.mk
 include $(AM_HOME)/am/arch/platform/nemu.mk
+
+AM_SRCS += riscv32/nemu/libint.c

am/src/riscv32/nemu/libint.c

@@ -0,0 +1,14 @@
+#include <klib.h>
+
+// for more details, refer to
+// https://gcc.gnu.org/onlinedocs/gccint/Integer-library-routines.html
+
+// only use for linking
+
+int __clzsi2 (unsigned int a) {
+  assert(0);
+}
+
+int __ctzsi2 (unsigned int a) {
+  assert(0);
+}