runtime: make concatstring{2,3,4,5} consistent w/ compiler's use

Internally the compiler uses a0,a1,a3 string, not [3]string,
and this lead to different parameter passing (memory, versus registers)
which of course did not work.

Updates #40724.

Change-Id: I1dbf479b88134559ba54b4b00a042b9a0fd128b8
Reviewed-on: https://go-review.googlesource.com/c/go/+/306910


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