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.

src/crypto/rand/rand_windows.go

@@ -1,6 +1,9 @@
 package rand
 
-import "errors"
+import (
+	"errors"
+	"unsafe"
+)
 
 func init() {
 	Reader = &reader{}
@@ -16,28 +19,22 @@ func (r *reader) Read(b []byte) (n int, err error) {
 		return
 	}
 
-	var randomByte uint32
-	for i := range b {
-		// Call rand_s every four bytes because it's a C int (always 32-bit in
-		// Windows).
-		if i%4 == 0 {
-			errCode := libc_rand_s(&randomByte)
-			if errCode != 0 {
-				// According to the documentation, it can return an error.
-				return n, errRandom
-			}
-		} else {
-			randomByte >>= 8
-		}
-		b[i] = byte(randomByte)
+	// Use the old RtlGenRandom, introduced in Windows XP.
+	// Even though the documentation says it is deprecated, it is widely used
+	// and probably won't go away anytime soon.
+	// See for example: https://github.com/golang/go/issues/33542
+	// For Windows 7 and newer, we might switch to ProcessPrng in the future
+	// (which is a documented function and might be a tiny bit faster).
+	ok := libc_RtlGenRandom(unsafe.Pointer(&b[0]), len(b))
+	if !ok {
+		return 0, errRandom
 	}
-
 	return len(b), nil
 }
 
-// Cryptographically secure random number generator.
-// https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/rand-s?view=msvc-170
-// errno_t rand_s(unsigned int* randomValue);
+// This function is part of advapi32.dll, and is called SystemFunction036 for
+// some reason. It's available on Windows XP and newer.
+// See: https://learn.microsoft.com/en-us/windows/win32/api/ntsecapi/nf-ntsecapi-rtlgenrandom
 //
-//export rand_s
-func libc_rand_s(randomValue *uint32) int32
+//export SystemFunction036
+func libc_RtlGenRandom(buf unsafe.Pointer, len int) bool