[dev.typeparams] cmd/compile: functions to create GC shape types/names for a concrete type

Created functions to create GC shape type and names, based on a proposal
from Keith. Kept unsigned and signed integer types as different, since
they have different shift operations.

Included adding in alignment fields where padding is
required between fields, even though that seems like it will be fairly
uncommon to use.

Added some extra unusual struct typeparams (for testing the gcshape
names/types) in index.go test.

Change-Id: I8132bbd28098bd933435b8972ac5cc0b39f4c0df
Reviewed-on: https://go-review.googlesource.com/c/go/+/329921


Trust: Dan Scales <danscales@google.com>
Run-TryBot: Dan Scales <danscales@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Keith Randall <khr@golang.org>