ARM: fix warning: target CPU does not support interworking

This patch fixes warnings like this:

start.S:0: warning: target CPU does not support interworking

which come from some ARM cross compilers and are caused by hard-coded
(with "--with-cpu=arm9" configuration option) ARM targets (which
support ARM Thumb instructions), while the ARM target selected from
the command line (with "-march=armv4") doesn't support Thumb
instructions.

This warning is issued by the compiler regardless of the real use of
the Thumb instructions in code.

To fix this problem, we use options according to compiler version
being used.

Signed-off-by: Sergei Poselenov <sposelenov@emcraft.com>
Signed-off-by: Wolfgang Denk <wd@denx.de>