builder: build wasi-libc inside TinyGo

Instead of relying on a build when TinyGo is being built, do it like all
other libraries when it is needed.

This brings a few benefits:

  * No more running `make wasi-libc` on the command line as a special
    case for WebAssembly. The generic `git submodule update --init` step
    is now enough for wasi-libc.
  * It becomes much easier to customize the build per system. For
    example: include/exclude malloc as needed, disable/enable bulk
    memory operations per target, etc.

compileopts/config.go

@@ -371,10 +371,10 @@ func (c *Config) LibcCFlags() []string {
 			"-isystem", filepath.Join(root, "lib", "musl", "include"),
 		}
 	case "wasi-libc":
-		root := goenv.Get("TINYGOROOT")
+		path := c.LibcPath("wasi-libc")
 		return []string{
 			"-nostdlibinc",
-			"-isystem", root + "/lib/wasi-libc/sysroot/include",
+			"-isystem", filepath.Join(path, "include"),
 		}
 	case "wasmbuiltins":
 		// nothing to add (library is purely for builtins)