RISC-V: Add initial return value location support.

Started with the aarch64 support and modified it for RISC-V.  The
flattened structure support hasn't been written yet, but the rest of
it should be correct for the LP64D ABI.  We have potentially 6 different
ABIs to support, so this requires checking elf header flags in riscv_init
when setting the hook.

Signed-off-by: Jim Wilson <jimw@sifive.com>