aarch64: Add bytes_below_hard_fp to frame info

aarch64: Add bytes_below_hard_fp to frame info

Following on from the previous bytes_below_saved_regs patch, this one
records the number of bytes that are below the hard frame pointer.
This eventually replaces below_hard_fp_saved_regs_size.

If a frame pointer is not needed, the epilogue adds final_adjust
to the stack pointer before restoring registers:

     aarch64_add_sp (tmp1_rtx, tmp0_rtx, final_adjust, true);

Therefore, if the epilogue needs to restore the stack pointer from
the hard frame pointer, the directly corresponding offset is:

     -bytes_below_hard_fp + final_adjust

i.e. go from the hard frame pointer to the bottom of the frame,
then add the same amount as if we were using the stack pointer
from the outset.

gcc/
	* config/aarch64/aarch64.h (aarch64_frame::bytes_below_hard_fp): New
	field.
	* config/aarch64/aarch64.cc (aarch64_layout_frame): Initialize it.
	(aarch64_expand_epilogue): Use it instead of
	below_hard_fp_saved_regs_size.