As pointed out in ARM-software#253
the R_AARCH64_AUTH_GOT_LO12_NC is meant to be the AUTH variant of
R_AARCH64_LD64_GOT_LO12_NC. As there is also a
R_AARCH64_LD32_GOT_LO12_NC relocation rename the relocation to
R_AARCH64_LD64_AUTH_GOT_LO12_NC.

These relocations are in the appendix as we are currently
expecting the GOT to be RELRO and unsigned in most signing schemas.

There is no equivalent for this relocation in the standard ABI
it is used by runtime code to calculate the address of
a GOT slot so it can be used as one of the inputs to an
authenticate instruction. Add a note that this matches up
with the :got_auth_lo12: operator for future reference.

Part of ARM-software#253