sh: Provide a FLAT_PLAT_INIT() definition.

SH needs this in order to make sure that r4 has a sane value at process
entry time, which the libc expects has already been taken care of.

Fixes random crashes in flat binaries.

Signed-off-by: Takashi YOSHII <yoshii.takashi@renesas.com>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>