An entry function should begin with a exception header. For this to work
properly the entry function should not contain any code which gcc might
put before the header. To make this sure change the ENTRY_FUNCTION macro
so that it generates one function which only contains the exception header
and a second function which contains the original body of the entry function.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>