[PAUTHABIELF64] Add R_AARCH64_AUTH_GOT_ADR_PREL_LO21 relocation

With the tiny code model and a signed GOT, an adr instruction is
needed to get the address of the GOT entry for input to the
authenication. For example:

adr x8, :got_auth: symbol
ldr x0, [x8]
// Authenticate to get unsigned pointer
autia x0, x8

The adr requires a new relocation code where there isn't a direct
equivalent in the main ABI as there is not need to take the
address of the GOT slot when no authentication is required.

We define R_AARCH64_AUTH_GOT_ADR_PREL21_LO21 for this purpose
following the naming convention of R_<CLS>_ADR_PREL_LO21. which is
its closest equivalent.

pauthabielf64/pauthabielf64.rst

@@ -1079,7 +1079,7 @@ GOT entry as the modifier. The static linker must encode the signing
 schema into the GOT slot. AUTH variant dynamic relocations must be
 used for signed GOT entries.
 
-Example Code to access a signed GOT entry
+Example Code to access a signed GOT entry with the small code model:
 
 .. code-block:: asm
 
@@ -1092,6 +1092,18 @@ Example Code to access a signed GOT entry
 In the example the :got_auth: and :got_auth_lo12: operators result in
 AUTH variant GOT generating relocations being used.
 
+Example Code to access a signed GOT entry with the tiny code model:
+
+.. code-block:: asm
+
+  adr x8, :got_auth: symbol
+  ldr x0, [x8]
+  // Authenticate to get unsigned pointer
+  autia x0, x8
+
+Compared to the tiny code model without pointer authentication an
+additonal adr is required to get the address of the GOT entry.
+
 AUTH variant GOT Generating Relocations
 ---------------------------------------
 
@@ -1167,6 +1179,11 @@ The GOT entries must be relocated by AUTH variant dynamic relocations.
   |             |                                        |                                  | value to bits [11:0] of  |
   |             |                                        |                                  | X. No overflow check.    |
   +-------------+----------------------------------------+----------------------------------+--------------------------+
+  | 0x811D      | R\AARCH64\_AUTH\_GOT\_ADR\_PREL\_LO21  | G(ENCD(GDAT(S + A))) - P         | Set the immediate        |
+  |             |                                        |                                  | value to bits[20:0] of X;|
+  |             |                                        |                                  | check that -2 :sup:`20`  |
+  |             |                                        |                                  | <= 2 :sup: `20`          |
+  +-------------+----------------------------------------+----------------------------------+--------------------------+
 
 .. raw:: pdf
 
@@ -1181,7 +1198,8 @@ is the PAuth ABI equivalent of ``R_AARCH64_RELATIVE``. The underlying
 calculation performed by the dynamic linker is the same, the only
 difference is that the resulting pointer is signed. The dynamic linker
 reads the signing schema from the contents of the place of the dynamic
-relocation.
+relocation. The ``R_AARCH64_AUTH_GOT_ADR_PREL_LO21`` relocation is used
+with the ``:got_auth:`` operator on an adr instruction.
 
 .. table:: Additional AUTH Dynamic relocations