[AArch64] Use frame reference in aarch64_layout_frame

[AArch64] Use frame reference in aarch64_layout_frame

Using the full path "cfun->machine->frame" in aarch64_layout_frame
led to awkward formatting in some follow-on patches, so it seemed
worth using a local reference instead.

2019-10-16  Richard Sandiford  <richard.sandiford@arm.com>

gcc/
	* config/aarch64/aarch64.c (aarch64_layout_frame): Use a local
	"frame" reference instead of always referring directly to
	"cfun->machine->frame".

git-svn-id: svn+ssh://gcc.gnu.org/svn/gcc/trunk@277059 138bc75d-0d04-0410-961f-82ee72b054a4