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.

compileopts/config.go

@@ -23,8 +23,9 @@ import (
 // builder.Library struct but that's hard to do since we want to know the
 // library path in advance in several places).
 var libVersions = map[string]int{
-	"musl":  3,
-	"bdwgc": 2,
+	"musl":         3,
+	"bdwgc":        2,
+	"wasmbuiltins": 1,
 }
 
 // Config keeps all configuration affecting the build in a single struct.