sh: Add ashrsi3 libgcc function

The ashrsi3 function is used by some commands that aren't in SH2A
default configs (e.g. JFFS2).

The ashrsi3.S file has been copied from Linux.

Signed-off-by: Phil Edworthy <phil.edworthy@renesas.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>