builder, loader: link .syso files in Go package directories

Among other things, this enables WebAssembly programs to link in custom sections,
which can be stored in a relocatable wasm module with a .syso suffix in a package directory.

This will simplify the build process for WASI 0.2, removing the need to run
'wasm-tools component embed' with the original source WIT files.

builder/build.go

@@ -701,6 +701,12 @@ func Build(pkgName, outpath, tmpdir string, config *compileopts.Config) (BuildRe
 			}
 			linkerDependencies = append(linkerDependencies, job)
 		}
+
+		// Add .syso files
+		// TODO: is this the right way to do this?
+		for _, filename := range pkg.SysoFiles {
+			ldflags = append(ldflags, filepath.Join(pkg.Dir, filename))
+		}
 	}
 
 	// Linker flags from CGo lines:

loader/loader.go

@@ -60,9 +60,10 @@ type PackageJSON struct {
 	}
 
 	// Source files
-	GoFiles  []string
-	CgoFiles []string
-	CFiles   []string
+	GoFiles   []string
+	CgoFiles  []string
+	CFiles    []string
+	SysoFiles []string
 
 	// Embedded files
 	EmbedFiles []string