wasm: add //go:wasmexport support (#4451)

This adds support for the `//go:wasmexport` pragma as proposed here:
https://github.com/golang/go/issues/65199

It is currently implemented only for wasip1 and wasm-unknown, but it is
certainly possible to extend it to other targets like GOOS=js and
wasip2.

targets/wasm-unknown.json

@@ -3,6 +3,7 @@
 	"cpu":           "generic",
 	"features":      "+mutable-globals,+nontrapping-fptoint,+sign-ext,-bulk-memory",
 	"build-tags":    ["tinygo.wasm", "wasm_unknown"],
+	"buildmode":    "c-shared",
 	"goos":          "linux",
 	"goarch":        "arm",
 	"linker":        "wasm-ld",