[Morello] Record mapping symbols and mark C64 function symbols as special

[Morello] Record mapping symbols and mark C64 function symbols as special

This patch teaches GDB about AArch64 mapping symbols and special minimal
symbols.

FIXME-Morello: This is currently not actively used, but will be used to detect
capability mode functions later.

gdb/ChangeLog:

2020-10-20  Luis Machado  <luis.machado@arm.com>

	* aarch64-tdep.c: Include elf-bfd.h.
	(MSYMBOL_SET_SPECIAL, MSYMBOL_IS_SPECIAL): New constants.
	(aarch64_mapping_symbol): New struct.
	(aarch64_mapping_symbol_vec): New typedef.
	(aarch64_per_bfd): New struct.
	(aarch64_bfd_data_key): New static global.
	(aarch64_elf_make_msymbol_special, +aarch64_record_special_symbol): New
	function.
	(aarch64_gdbarch_init): Register hooks.