binfmt_flat: Stub in a FLAT_PLAT_INIT().

This provides a FLAT_PLAT_INIT() arch hook for platforms that need to set
up specific register state prior to calling in to the process, as per
ELF_PLAT_INIT().

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