aarch64: Rename hard_fp_offset to bytes_above_hard_fp

aarch64: Rename hard_fp_offset to bytes_above_hard_fp

Similarly to the previous locals_offset patch, hard_fp_offset
was described as:

  /* Offset from the base of the frame (incomming SP) to the
     hard_frame_pointer.  This value is always a multiple of
     STACK_BOUNDARY.  */
  poly_int64 hard_fp_offset;

which again took an “upside-down” view: higher offsets meant lower
addresses.  This patch renames the field to bytes_above_hard_fp instead.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::hard_fp_offset): Rename
	to...
	(aarch64_frame::bytes_above_hard_fp): ...this.
	* config/aarch64/aarch64.cc (aarch64_layout_frame)
	(aarch64_expand_prologue): Update accordingly.
	(aarch64_initial_elimination_offset): Likewise.