[aaelf64-morello] Document program headers and dynamic section (#359)

PT_AARCH64_MEMTAG_CHERI has been used by GDB and CheriBSD for several
years at this point, after discussion with Arm and Linaro, but was never
officially documented. Thankfully it appears that nothing has taken this
value in the meantime.

No Morello-specific dynamic tags exist.

Author: jrtc27

aaelf64-morello/aaelf64-morello.rst

@@ -1023,6 +1023,31 @@ the dynamic linker generates the table itself based on the ``R_MORELLO_GLOB_DAT`
 ``R_MORELLO_JUMP_SLOT`` relocations created by the static linker. The dynamic linker
 writes the generated capabilities back into the ``.got`` entry.
 
+Program Header
+--------------
+
+A Morello toolchain can emit segments in accordance to [CHERI_ELF_]. The
+following fields have Morello-specific meanings.
+
+p\_type
+  The below table lists the Morello-specific segment types.
+
+.. table:: Morello-specific segment types
+
+    +-----------------------------+------------+------------------------------------------------------+
+    | Name                        | p\_type    | Meaning                                              |
+    +=============================+============+======================================================+
+    | ``PT_AARCH64_MEMTAG_CHERI`` | 0x70000003 | Reserved for capability tag data dumps in core files |
+    +-----------------------------+------------+------------------------------------------------------+
+
+``PT_AARCH64_MEMTAG_CHERI`` segments (if present) hold capability tags for a
+particular memory range, packed as one bit per tag.
+
+Dynamic Section
+---------------
+
+A Morello toolchain can emit dynamic tags in accordance to [CHERI_ELF_].
+
 APPENDIX
 ========
 The status of this appendix is ``informative``.