[Morello] Capability support for exception headers

[Morello] Capability support for exception headers

- Identify and mark C64 frames
- Identify C64 registers including DDC.
- Identify 'purecap' argument to .cfi_startproc for C64 frames
- Emit 'C' in augmentation string for C64 frames
- Recognise the 'C' in the CIE augmentation string when parsing
  exception headers

Difference from LLVM: The llvm assembler only uses purecap to add C to
the augmentation string.  The GNU assembler on the other hand uses
-march and validates that purecap is passed to .cfi_startproc only for
-morello+c64.  This means that for code compiled for A64, if llvm sees
`.cfi_startproc purecap`, it sets 'C' whereas the GNU assembler flags
an error.

bfd/ChangeLog:

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

	* elf-bfd.h (elf_backend_data): New callback
	elf_backend_eh_frame_augmentation_char.
	* elf-eh-frame.c (_bfd_elf_parse_eh_frame): Use it.
	* elfnn-aarch64.c (elf64_aarch64_eh_frame_augmentation_char):
	New function.
	(elf_backend_eh_frame_augmentation_char): New macro.

	* elfxx-target.h [!elf_backend_eh_frame_augmentation_char]:
	Set elf_backend_eh_frame_augmentation_char to NULL.
	(elfNN_bed): Initialise
	elf_backend_eh_frame_augmentation_char.

binutils/ChangeLog:

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

	* dwarf.c (dwarf_regnames_aarch64): Add capability registers.

gas/ChangeLog:

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

	* config/tc-aarch64.c (REG_DW_CSP, REG_DW_CLR): New macros.
	(s_aarch64_cfi_b_key_frame): Adjust for new entry_extras
	struct.
	(tc_aarch64_frame_initial_instructions): Adjust for C64.
	(tc_aarch64_fde_entry_init_extra,
	tc_aarch64_cfi_startproc_exp): New functions.
	(tc_aarch64_regname_to_dw2regnum): Support capability
	registers.
	* config/tc-aarch64.h (fde_entry): Forward declaration.
	(eh_entry_extras): New struct.
	(tc_fde_entry_extras, tc_cie_entry_extras): Use it.
	(tc_fde_entry_init_extra): Set to
	tc_aarch64_fde_entry_init_extra.
	(tc_output_cie_extra): Emit 'C' for C64.
	(tc_cie_fde_equivalent_extra): Adjust for C64.
	(tc_cie_entry_init_extra): Likewise.
	(tc_cfi_startproc_exp): New macro.
	(tc_aarch64_cfi_startproc_exp,
	tc_aarch64_fde_entry_init_extra): New function declarations.
	* dw2gencfi.c (tc_cfi_startproc_exp): New macro.
	(dot_cfi_startproc): Use it.
	* testsuite/gas/aarch64/morello-eh.d: New test.
	* testsuite/gas/aarch64/morello-eh.s: New test.