windows: use MSVCRT.DLL instead of UCRT on i386

This allows the binaries to run on Windows XP, without needing any extra
DLLs. Tested in an x86 Windows XP SP3 virtual machine.

compileopts/config.go

@@ -398,15 +398,25 @@ func (c *Config) LibcCFlags() []string {
 	case "mingw-w64":
 		root := goenv.Get("TINYGOROOT")
 		path := c.LibraryPath("mingw-w64")
-		return []string{
+		cflags := []string{
 			"-nostdlibinc",
 			"-isystem", filepath.Join(path, "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "crt"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "include"),
 			"-isystem", filepath.Join(root, "lib", "mingw-w64", "mingw-w64-headers", "defaults", "include"),
-			"-D_UCRT",
-			"-D_WIN32_WINNT=0x0a00", // target Windows 10
 		}
+		if c.GOARCH() == "386" {
+			cflags = append(cflags,
+				"-D__MSVCRT_VERSION__=0x700", // Microsoft Visual C++ .NET 2002
+				"-D_WIN32_WINNT=0x0501",      // target Windows XP
+			)
+		} else {
+			cflags = append(cflags,
+				"-D_UCRT",
+				"-D_WIN32_WINNT=0x0a00", // target Windows 10
+			)
+		}
+		return cflags
 	case "":
 		// No libc specified, nothing to add.
 		return nil

compileopts/target.go

@@ -437,6 +437,8 @@ func defaultTarget(options *Options) (*TargetSpec, error) {
 		case "386":
 			spec.LDFlags = append(spec.LDFlags,
 				"-m", "i386pe",
+				"--major-os-version", "4",
+				"--major-subsystem-version", "4",
 			)
 			// __udivdi3 is not present in ucrt it seems.
 			spec.RTLib = "compiler-rt"