all: add support for windows/amd64

This uses Mingw-w64, which seems to be the de facto standard for porting
Unixy programs to Windows.

src/crypto/rand/rand_windows.go

@@ -0,0 +1,42 @@
+package rand
+
+import "errors"
+
+func init() {
+	Reader = &reader{}
+}
+
+type reader struct {
+}
+
+var errRandom = errors.New("failed to obtain random data from rand_s")
+
+func (r *reader) Read(b []byte) (n int, err error) {
+	if len(b) == 0 {
+		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)
+	}
+
+	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);
+//export rand_s
+func libc_rand_s(randomValue *uint32) int32