wasm: add support for GOOS=wasip1

This adds true GOOS=wasip1 support in addition to our existing
-target=wasi support. The old support for WASI isn't removed, but should
be treated as deprecated and will likely be removed eventually to reduce
the test burden.

src/testing/is_wasi_no_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
-//go:build !wasi
+//go:build !wasi && !wasip1
 
 package testing_test
 

src/testing/is_wasi_test.go

@@ -2,7 +2,7 @@
 // Use of this source code is governed by a BSD-style
 // license that can be found in the LICENSE file.
 //
-//go:build wasi
+//go:build wasi || wasip1
 
 package testing_test