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>

src/runtime/string.go

@@ -55,20 +55,20 @@ func concatstrings(buf *tmpBuf, a []string) string {
 	return s
 }
 
-func concatstring2(buf *tmpBuf, a [2]string) string {
-	return concatstrings(buf, a[:])
+func concatstring2(buf *tmpBuf, a0, a1 string) string {
+	return concatstrings(buf, []string{a0, a1})
 }
 
-func concatstring3(buf *tmpBuf, a [3]string) string {
-	return concatstrings(buf, a[:])
+func concatstring3(buf *tmpBuf, a0, a1, a2 string) string {
+	return concatstrings(buf, []string{a0, a1, a2})
 }
 
-func concatstring4(buf *tmpBuf, a [4]string) string {
-	return concatstrings(buf, a[:])
+func concatstring4(buf *tmpBuf, a0, a1, a2, a3 string) string {
+	return concatstrings(buf, []string{a0, a1, a2, a3})
 }
 
-func concatstring5(buf *tmpBuf, a [5]string) string {
-	return concatstrings(buf, a[:])
+func concatstring5(buf *tmpBuf, a0, a1, a2, a3, a4 string) string {
+	return concatstrings(buf, []string{a0, a1, a2, a3, a4})
 }
 
 // slicebytetostring converts a byte slice to a string.