Fixed panda "make recover" undefined reference __aeabi_llsr (#522)

In v0.5.8, panda.bin compile optimization flag was changed from O2 to
Os to keep its size below 32768. However, the compiler also optimized
away __aeabi_llsr (long long right shift function). The fix is to
manually link libgcc.a

An alternative fix is to use O1 instead of Os. But that might be less
long term solution.

Resolves: #522

panda/board/build.mk

@@ -55,7 +55,7 @@ obj/$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/main.$(PROJ_NAME).o
 
 
 obj/bootstub.$(PROJ_NAME).bin: obj/$(STARTUP_FILE).o obj/bootstub.$(PROJ_NAME).o obj/sha.$(PROJ_NAME).o obj/rsa.$(PROJ_NAME).o
-	$(CC) $(CFLAGS) -o obj/bootstub.$(PROJ_NAME).elf $^
+	$(CC) $(CFLAGS) -o obj/bootstub.$(PROJ_NAME).elf $^ -lgcc
 	$(OBJCOPY) -v -O binary obj/bootstub.$(PROJ_NAME).elf $@
 
 clean: