The C entry function isn't naked, so tries to push to the stack
in the function prologue. This doesn't work on QEMU where there
is no valid SP on entry. Convert the lowlevel entry to the
assembly facilities provided for this case.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>