[PAUTHABIELF64] Introduce `R_AARCH64_AUTH_GOT_ADR_PREL21` relocation

[kovdan01]

It looks like that R_AARCH64_AUTH_GOT_LD_PREL19 described at https://github.com/ARM-software/abi-aa/blob/main/pauthabielf64/pauthabielf64.rst#auth-variant-got-generating-relocations is not enough for tiny code model in case of signed GOT. Since the address of the GOT slot is used as a modifier, we need to store that address somewhere. To take this address, we should probably use adr instruction with 21-bit immediate.

I suggest to add a R_AARCH64_AUTH_GOT_ADR_PREL21 relocation which would correspond to the following assembly operator adr x16, :got_auth:sym.