[Morello] Add symbol markers for reloc section for static binaries

[Morello] Add symbol markers for reloc section for static binaries

Add symbols __cap_dynrelocs_start and __cap_dynrelocs_end to mark the
start and end of the .rela.dyn section when building a static
executable without PIE.  This allows the runtime startup to traverse
the section and initialise capabilities without having to read the ELF
headers.

All relocations must be of type R_C64_RELATIVE and have the following
properties:

- Frag contains the base of the capability to be initialised
- Frag + 8 has the size and permissions encoded into 56 and 8 bits
  respectively
- Addend is the offset from the capability base

bfd/ChangeLog:

2020-10-20  Siddhesh Poyarekar  <siddesh.poyarekar@arm.com>

	* elfnn-aarch64.c (elfNN_aarch64_final_link_relocate): Emit
	relative C64 relocations for static binaries early.
	(aarch64_elf_init_got_section): Add capability relocations to
	SRELCAPS for non-PIE static binaries.
	(elfNN_aarch64_allocate_dynrelocs): Likewise.
	(elfNN_aarch64_always_size_sections): Emit
	__cap_dynrelocs_start and __cap_dynrelocs_end.