runtime: clean up runtime.call* frame sizes on ARM64

ARM64 used to require that all assembly frame sizes were of the form
16*N+8 because ARM64 requires 16-byte SP alignment and the assembler
added an 8 byte LR slot. This made all of the runtime.call* frame
sizes wonky. The assembler now rounds up the frame size appropriately
after adding any additional slots it needs, so this is no longer
necessary.

This CL cleans up the frame sizes of these functions so they look the
way you'd expect and match all other architectures.

Change-Id: I47819092296b8983c43eadf2e66c7c1e0d518555
Reviewed-on: https://go-review.googlesource.com/c/go/+/259448


Trust: Austin Clements <austin@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>