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.

builder/builtins.go

@@ -3,6 +3,7 @@ package builder
 import (
 	"os"
 	"path/filepath"
+	"strings"
 
 	"github.com/tinygo-org/tinygo/compileopts"
 	"github.com/tinygo-org/tinygo/goenv"
@@ -201,6 +202,11 @@ var avrBuiltins = []string{
 	"avr/udivmodqi4.S",
 }
 
+// Builtins needed specifically for windows/386.
+var windowsI386Builtins = []string{
+	"i386/chkstk.S", // also _alloca
+}
+
 // libCompilerRT is a library with symbols required by programs compiled with
 // LLVM. These symbols are for operations that cannot be emitted with a single
 // instruction or a short sequence of instructions for that target.
@@ -229,6 +235,10 @@ var libCompilerRT = Library{
 			builtins = append(builtins, avrBuiltins...)
 		case "x86_64", "aarch64", "riscv64": // any 64-bit arch
 			builtins = append(builtins, genericBuiltins128...)
+		case "i386":
+			if strings.Split(target, "-")[2] == "windows" {
+				builtins = append(builtins, windowsI386Builtins...)
+			}
 		}
 		return builtins, nil
 	},