compiler,runtime: fix multiple definitions of a single function

strings.IndexByte was implemented in the runtime up to Go 1.11. It is
implemented using a direct call to internal/bytealg.IndexByte since Go
1.12.

Make sure we remain compatible with both.

src/runtime/strings_go111.go

@@ -0,0 +1,12 @@
+// +build !go1.12
+
+package runtime
+
+// indexByte provides compatibility with Go 1.11.
+// See the following:
+// https://github.com/tinygo-org/tinygo/issues/351
+// https://github.com/golang/go/commit/ad4a58e31501bce5de2aad90a620eaecdc1eecb8
+//go:linkname indexByte strings.IndexByte
+func indexByte(s string, c byte) int {
+	return indexByteString(s, c)
+}