cmd/compile: use canonical stringslice/ representations in abiutils

A chunk of code in abiutils was synthesizing the internals of a Go
string type as "struct { unsafe.Pointer, uintptr }" instead of the
more canonical representation "struct { *uint8, int }" used elsewhere
in the compiler. The abiutils type was being pulled into the code
during late call expansion, which resulted in two different entries in
the SSA named value table for the same variable piece, each with
different types; this then confused DWARF location list generation.
This patch changes the abiutils synthesized type to be consistent with
other parts of the back end, and makes a similar change for
synthesized slice types (use "struct { *uint8, int, int }").

Fixes #47354.

Change-Id: If789031cdc7abaf215bc75ee6eb863defbe530be
Reviewed-on: https://go-review.googlesource.com/c/go/+/362715


Trust: Than McIntosh <thanm@google.com>
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>