riscv: fix accessing 8-byte variable from RV32

A memory save operation to 8-byte variable in RV32 is divided into
two sw instructions in the put_user macro.  The current fixup returns
execution flow to the second sw instead of the one after it.

This patch fixes this fixup code according to the load access part.

Signed-off-by: Alan <Kao&lt;alankao@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Cc: Vincent Chen <deanbo422@gmail.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>