aarch64: Tweak aarch64_save/restore_callee_saves

aarch64: Tweak aarch64_save/restore_callee_saves

aarch64_save_callee_saves and aarch64_restore_callee_saves took
a parameter called start_offset that gives the offset of the
bottom of the saved register area from the current stack pointer.
However, it's more convenient for later patches if we use the
bottom of the entire frame as the reference point, rather than
the bottom of the saved registers.

Doing that removes the need for the callee_offset field.
Other than that, this is not a win on its own.  It only really
makes sense in combination with the follow-on patches.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::callee_offset): Delete.
	* config/aarch64/aarch64.cc (aarch64_layout_frame): Remove
	callee_offset handling.
	(aarch64_save_callee_saves): Replace the start_offset parameter
	with a bytes_below_sp parameter.
	(aarch64_restore_callee_saves): Likewise.
	(aarch64_expand_prologue): Update accordingly.
	(aarch64_expand_epilogue): Likewise.