sh: Fix up SH-X3 general exception handler build.

With the recent entry.S refactoring, the SH-X3 path had a mov.l for a
register to register copy, resulting in:

  AS      arch/sh/kernel/cpu/sh4/../sh3/entry.o
arch/sh/kernel/cpu/sh4/../sh3/entry.S: Assembler messages:
arch/sh/kernel/cpu/sh4/../sh3/entry.S:366: Error: invalid operands for opcode
make[3]: *** [arch/sh/kernel/cpu/sh4/../sh3/entry.o] Error 1

Switch it over to a mov to fix it up.

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