openrisc: Add support for more module relocations

When testing modules in OpenRISC I found R_OR1K_AHI16 (signed adjusted
high 16-bit) and R_OR1K_SLO16 (split low 16-bit) relocations are used in
modules but not implemented yet.

This patch implements the relocations. I have tested with a few modules.

Signed-off-by: Stafford Horne <shorne@gmail.com>