builder: add strlen to wasm builtins

LLVM 22 can replace string-scanning loops with calls to strlen during
optimization. Add the implementation to wasmbuiltins and invalidate
cached archives.

Add an optimized wasm-unknown smoke test that exercises the reflect
name-decoding path which exposed the missing symbol.

testdata/wasm-unknown-opt/main.go

@@ -0,0 +1,15 @@
+package main
+
+import "reflect"
+
+type value struct {
+	Field int
+}
+
+//go:wasmexport typeNameLength
+func typeNameLength() uint32 {
+	return uint32(len(reflect.TypeOf(value{}).String()))
+}
+
+func main() {
+}