or1k: Implement relocation R_OR1K_GOT_AHI16 for gotha()
The gotha() relocation mnemonic will be outputted by OpenRISC GCC when
using the -mcmodel=large option. This relocation is used along with
got() to generate 32-bit GOT offsets. This increases the previous GOT
offset limit from the previous 16-bit (64K) limit.
This is needed on large binaries where the GOT grows larger than 64k.