runtime: only implement CountString for required platforms

src/runtime/string.go

@@ -216,16 +216,3 @@ func indexByteString(s string, c byte) int {
 	}
 	return -1
 }
-
-// countString copies the implementation from
-// https://github.com/golang/go/blob/67f181bfd84dfd5942fe9a29d8a20c9ce5eb2fea/src/internal/bytealg/count_generic.go#L1
-//go:linkname countString internal/bytealg.CountString
-func countString(s string, c byte) int {
-	n := 0
-	for i := 0; i < len(s); i++ {
-		if s[i] == c {
-			n++
-		}
-	}
-	return n
-}

src/runtime/string_count.go

@@ -0,0 +1,23 @@
+// +build amd64 arm,go1.13 arm64 ppc64le ppc64 s390x
+
+package runtime
+
+// This file implements the string counting functions used by the strings
+// package, for example. It must be reimplemented here as a replacement for the
+// Go stdlib asm implementations, but only when the asm implementations are used
+// (this varies by Go version).
+// Track this file for updates:
+// https://github.com/golang/go/blob/master/src/internal/bytealg/count_native.go
+
+// countString copies the implementation from
+// https://github.com/golang/go/blob/67f181bfd84dfd5942fe9a29d8a20c9ce5eb2fea/src/internal/bytealg/count_generic.go#L1
+//go:linkname countString internal/bytealg.CountString
+func countString(s string, c byte) int {
+	n := 0
+	for i := 0; i < len(s); i++ {
+		if s[i] == c {
+			n++
+		}
+	}
+	return n
+}