[GDBserver][AArch64] Cleanup comments for each linux_target_ops method

[GDBserver][AArch64] Cleanup comments for each linux_target_ops method

This patch cleans up the comments for each linux_target_ops methods.  We
should mention which method each function implements but there is no
need to duplicate information already mentionned in the base target_ops
or linux_target_ops definitions.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c: Add comments for each linux_target_ops
	method.  Remove comments already covered in target_ops and
	linux_target_ops definitions.
	(the_low_target): Add comments for each unimplemented method.