cmd/compile: handle field padding for register-passed structs

When constructing multi-piece DWARF location expressions for
struct-typed parameters using the register ABI, make sure that the
location expressions generated properly reflect padding between
elements (this is required by debuggers). Example:

   type small struct { x uint16 ; y uint8 ; z int32 }
   func ABC(p1 int, p2 small, f1 float32) {
     ...

In the DWARF location expression for "p2" on entry to the routine, we
need pieces for each field, but for debuggers (such as GDB) to work
properly, we also need to describe the padding between elements. Thus
instead of

  <rbx> DW_OP_piece 2 <rcx> DW_OP_piece 1 <rdi> DW_OP_piece 4

we need to emit

  <rbx> DW_OP_piece 2 <rcx> DW_OP_piece 1 DW_OP_piece 1 <rdi> DW_OP_piece 4

This patch adds a new helper routine in abiutils to compute the
correct padding amounts for a struct type, a unit test for the helper,
and updates the debug generation code to call the helper and insert
apadding "piece" ops in the right spots.

Updates #40724.
Updates #45720.

Change-Id: Ie208bee25776b9eb70642041869e65e4fa65a005
Reviewed-on: https://go-review.googlesource.com/c/go/+/315071


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: David Chase <drchase@google.com>