arm: the ARM920T is THUMB capable, so select the appropriate ISA

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

arch/arm/Makefile

@@ -9,7 +9,7 @@ endif
 
 # This selects which instruction set is used.
 arch-$(CONFIG_CPU_ARM720T)	=-march=armv4
-arch-$(CONFIG_CPU_ARM920T)	=-march=armv4
+arch-$(CONFIG_CPU_ARM920T)	=-march=armv4t
 arch-$(CONFIG_CPU_ARM926EJS)	=-march=armv5te
 arch-$(CONFIG_CPU_ARM946ES)	=-march=armv4
 arch-$(CONFIG_CPU_SA1100)	=-march=armv4