riscv: support standalone applications on RV64I systems

Add an implementation of EXPORT_FUNC() for RV64I systems to support them
in standalone applications.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>