[AArch64][ELF] Support R_AARCH64_AUTH_ABS64 static relocation

The patch adds parser, MCExpr, and emitter support for the authenticated
pointer auth relocation.

In assembly, this is expressed using:
  .quad <symbol>@AUTH(<key>, <discriminator> [, addr])
For example:
  .quad _g3@AUTH(ib, 1234, addr)

The optional 'addr' specifier represents whether the generated pointer
authentication code will also include address diversity (by blending the
address of the storage location of the relocated pointer with the
user-specified constant discriminator).

The @AUTH expression lowers to R_AARCH64_AUTH_ABS64 ELF relocation.

The signing schema is encoded in the place of relocation to be applied
as follows:

```
| 63                | 62 | 61:60 | 59:48 |  47:32        | 31:0   |
| ----------------- | -- | ----- | ----- | ------------- | ------ |
| address diversity | 0  | key   | 0     | discriminator | addend |
```

See the following for details:
https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst#static-relocations

Differential Revision: https://reviews.llvm.org/D156505



Co-authored-by: Ahmed Bougacha <ahmed@bougacha.org>
Co-authored-by: Peter Collingbourne <peter@pcc.me.uk>