aarch64: Rename locals_offset to bytes_above_locals

aarch64: Rename locals_offset to bytes_above_locals

locals_offset was described as:

  /* Offset from the base of the frame (incomming SP) to the
     top of the locals area.  This value is always a multiple of
     STACK_BOUNDARY.  */

This is implicitly an “upside down” view of the frame: the incoming
SP is at offset 0, and anything N bytes below the incoming SP is at
offset N (rather than -N).

However, reg_offset instead uses a “right way up” view; that is,
it views offsets in address terms.  Something above X is at a
positive offset from X and something below X is at a negative
offset from X.

Also, even on FRAME_GROWS_DOWNWARD targets like AArch64,
target-independent code views offsets in address terms too:
locals are allocated at negative offsets to virtual_stack_vars.

It seems confusing to have *_offset fields of the same structure
using different polarities like this.  This patch tries to avoid
that by renaming locals_offset to bytes_above_locals.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::locals_offset): Rename to...
	(aarch64_frame::bytes_above_locals): ...this.
	* config/aarch64/aarch64.cc (aarch64_layout_frame)
	(aarch64_initial_elimination_offset): Update accordingly.