compileopts: don't filter build tags, use specific build tags instead

This basically reverts https://github.com/tinygo-org/tinygo/pull/3357
and replaces it with a different mechanism to get to the same goal.

I do not think filtering tags like this is a good idea: it's the wrong
part of the compiler to be concerned with such tags (that part sets
tags, but doesn't modify existing tags). Instead, I've written the
//go:build lines in such a way that it has the same effect: WASI
defaults to leveldb, everything else defaults to fnv, and it's possible
to override the default using build tags.

targets/wasi.json

@@ -2,7 +2,7 @@
 	"llvm-target":   "wasm32-unknown-wasi",
 	"cpu":           "generic",
 	"features":      "+bulk-memory,+nontrapping-fptoint,+sign-ext",
-	"build-tags":    ["tinygo.wasm", "wasi", "runtime_memhash_leveldb"],
+	"build-tags":    ["tinygo.wasm", "wasi"],
 	"goos":          "linux",
 	"goarch":        "arm",
 	"linker":        "wasm-ld",