Support the following relocations and assembly operators:
- `R_AARCH64_AUTH_GOT_ADR_PREL_LO21` (`:got_auth:` for `adr`)
- `R_AARCH64_AUTH_GOT_LD_PREL19` (`:got_auth:` for `ldr`)
`LOADgotAUTH` pseudo-instruction is expanded to actual instruction
sequence like the following.
```
adr x16, :got_auth:sym
ldr x0, [x16]
autia x0, x16
```
Both SelectionDAG and GlobalISel are suppported. For FastISel, we fall
back to SelectionDAG.
Tests starting with 'ptrauth-' have corresponding variants w/o this
prefix.
