qemu-arm: Disable gcc LTO option for nlrthumb.c.

If LTO is enabled for nlrthumb.c then gcc optimises away the nlr_push_tail
function when in fact it is needed.  So disable this option for this file.

qemu-arm/Makefile

@@ -53,6 +53,9 @@ OBJ_TEST += $(addprefix $(BUILD)/, $(SRC_TEST_C:.c=.o))
 OBJ_TEST += $(addprefix $(BUILD)/, $(SRC_S:.s=.o))
 OBJ_TEST += $(BUILD)/tinytest.o
 
+# we can't use LTO for nlrthumb.c because it contains inline assembler
+$(PY_BUILD)/nlrthumb.o: CFLAGS += -fno-lto
+
 all: run
 
 run: $(BUILD)/firmware.elf