[dev.regabi] cmd/compile/internal: specify memory layout

This CL expands internal-abi.md to cover Go's memory layout rules and
then uses this to specify the calling convention more precisely.

Change-Id: Ifeef9e49d9ccc8c7333dec81bdd47b511b028469
Reviewed-on: https://go-review.googlesource.com/c/go/+/286073


Trust: Austin Clements <austin@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Cherry Zhang <cherryyz@google.com>