runtime/all: add implementation of bytealg.CountString to complete #424

Signed-off-by: Ron Evans <ron@hybridgroup.com>

src/runtime/string.go

@@ -216,3 +216,16 @@ 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
+}