arm: build some file(s) as ARM mode only

Signed-off-by: David Müller <d.mueller@elsoft.ch>

arch/arm/cpu/arm920t/Makefile

@@ -13,3 +13,9 @@ obj-$(CONFIG_USE_IRQ)	+= interrupts.o
 obj-$(CONFIG_EP93XX) += ep93xx/
 obj-$(CONFIG_IMX) += imx/
 obj-$(CONFIG_S3C24X0) += s3c24x0/
+
+# some files can only build in ARM mode
+
+ifdef CONFIG_SYS_THUMB_BUILD
+CFLAGS_cpu.o := -marm
+endif