linux-user: Fix target_stat and target_stat64 for OpenRISC

linux-user: Fix target_stat and target_stat64 for OpenRISC

OpenRISC uses the asm-generic versions of target_stat and
target_stat64, but it was incorrectly using the x86/ARM/etc version
due to a misplaced defined(TARGET_OPENRISC).  The previously unused
OpenRISC section of the ifdef ladder also defined an incorrect
target_stat and omitted the target_stat64 definition.  Fix
target_stat, provide target_stat64, and add a comment noting that
these are the asm-generic versions for the benefit of future ports.

Reviewed-by: Jia Liu <proljc@gmail.com>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>