Add checks for the NT_ARM_SVE section in a core file.

Add checks for the NT_ARM_SVE section in a core file.

The NT_ARM_SVE section is documented here:
  https://github.com/torvalds/linux/blob/master/Documentation/arm64/sve.txt
  * A NT_ARM_SVE note will be added to each coredump for each thread of the
   dumped process.  The contents will be equivalent to the data that would have
   been read if a PTRACE_GETREGSET of NT_ARM_SVE were executed for each thread
   when the coredump was generated.

	* elf.c (elfcore_grok_aarch_sve): New function.
	(elfcore_grok_note): Check for Aarch64 SVE.
	(elfcore_write_aarch_sve): New function.
	(elfcore_write_register_note): Check for Aarch64 SVE.
	* elf-bfd.h(elfcore_grok_aarch_sve): New declaration.