set up go/types.Config.Sizes according to GOARCH

Otherwise we miscalculate int sizes, type sizes, alignments, and so on.
Caught by the GOARCH=386 go test on CI, since the os package imports
internal/syscall/unix, which uses arch-dependent padding.

The different padding between our incorrect use of go/types
and the correct typechecking done by the compiler caused different
obfuscation of fields, as the struct types stringified differently,
and they are used as a hash salt for field name obfuscation.

main.go

@@ -1668,7 +1668,10 @@ func typecheck(pkgPath string, files []*ast.File, origImporter importerWithMap)
 		Instances:  make(map[*ast.Ident]types.Instance),
 	}
 	// TODO(mvdan): we should probably set types.Config.GoVersion from go.mod
-	origTypesConfig := types.Config{Importer: origImporter}
+	origTypesConfig := types.Config{
+		Importer: origImporter,
+		Sizes:    types.SizesFor("gc", sharedCache.GoEnv.GOARCH),
+	}
 	pkg, err := origTypesConfig.Check(pkgPath, fset, files, info)
 	if err != nil {
 		return nil, nil, fmt.Errorf("typecheck error: %v", err)
@@ -2367,8 +2370,8 @@ func flagSetValue(flags []string, name, value string) []string {
 
 func fetchGoEnv() error {
 	out, err := exec.Command("go", "env", "-json",
-		// Keep in sync with sharedCache.GoEnv.
-		"GOOS", "GOMOD", "GOVERSION", "GOROOT",
+		// Keep in sync with [sharedCacheType.GoEnv].
+		"GOOS", "GOARCH", "GOMOD", "GOVERSION", "GOROOT",
 	).CombinedOutput()
 	if err != nil {
 		// TODO: cover this in the tests.

shared.go

@@ -60,7 +60,8 @@ type sharedCacheType struct {
 	// Filled directly from "go env".
 	// Keep in sync with fetchGoEnv.
 	GoEnv struct {
-		GOOS string // i.e. the GOOS build target
+		GOOS   string // the GOOS build target
+		GOARCH string // the GOARCH build target
 
 		GOMOD     string
 		GOVERSION string

testdata/script/crossbuild.txtar

@@ -40,3 +40,13 @@ import "net/http"
 func main() {
 	http.ListenAndServe("", nil)
 }
+
+-- 32bit.go --
+//go:build arm
+
+package main
+
+// Will give "out of bounds" if we don't correctly set up types.Config.Sizes.
+const is64bit = ^uint(0) >> 63 // 0 for 32-bit hosts, 1 for 64-bit ones.
+var x [1]struct{}
+var _ = x[is64bit]