sh: Fix build in start.S

By commit 6d8962e8,
label of _start was not usable in start.S and build fail.
This change label from  _start to _sh_start.

----
arch/sh/cpu/sh4/libsh4.o: In function `_start':
(.text+0x204): multiple definition of `_start'
arch/sh/cpu/sh4/start.o:(.text+0x0): first defined here
----

Signed-off-by: Nobuhiro Iwamatsu <nobuhiro.iwamatsu.yj@renesas.com>