sh: Use proper SH-2A CFLAGS on newer compilers.

-m2 doesn't end up working particularly well when we've got a constrained
toolchain target. Switch to the same semantics used by SH-4A to attempt
to get it right. Spotted by Alex Song <songqf9@yahoo.ca>.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>