crypto/rand: switch to arc4random_buf

This doesn't have the potential blocking issue of the getentropy call
(which calls WASI random_get when using wasi-libc) and should therefore
be a lot faster.

For context, this is what the random_get documentation says:

> Write high-quality random data into a buffer. This function blocks
> when the implementation is unable to immediately provide sufficient
> high-quality random data. This function may execute slowly, so when
> large mounts of random data are required, it's advisable to use this
> function to seed a pseudo-random number generator, rather than to
> provide the random data directly.

src/crypto/rand/rand_arc4random.go

@@ -0,0 +1,30 @@
+// +build darwin freebsd tinygo.wasm
+
+// This implementation of crypto/rand uses the arc4random_buf function
+// (available on both MacOS and WASI) to generate random numbers.
+//
+// Note: arc4random_buf (unlike what the name suggets) does not use the insecure
+// RC4 cipher. Instead, it uses a high-quality cipher, varying by the libc
+// implementation.
+
+package rand
+
+import "unsafe"
+
+func init() {
+	Reader = &reader{}
+}
+
+type reader struct {
+}
+
+func (r *reader) Read(b []byte) (n int, err error) {
+	if len(b) != 0 {
+		libc_arc4random_buf(unsafe.Pointer(&b[0]), uint(len(b)))
+	}
+	return len(b), nil
+}
+
+// void arc4random_buf(void *buf, size_t buflen);
+//export arc4random_buf
+func libc_arc4random_buf(buf unsafe.Pointer, buflen uint)

src/crypto/rand/rand_getentropy.go

@@ -1,38 +0,0 @@
-// +build darwin freebsd tinygo.wasm
-
-// This implementation of crypto/rand uses the getentropy system call (available
-// on both MacOS and WASI) to generate random numbers.
-
-package rand
-
-import (
-	"errors"
-	"unsafe"
-)
-
-var errReadFailed = errors.New("rand: could not read random bytes")
-
-func init() {
-	Reader = &reader{}
-}
-
-type reader struct {
-}
-
-func (r *reader) Read(b []byte) (n int, err error) {
-	if len(b) != 0 {
-		if len(b) > 256 {
-			b = b[:256]
-		}
-		result := libc_getentropy(unsafe.Pointer(&b[0]), len(b))
-		if result < 0 {
-			// Maybe we should return a syscall.Errno here?
-			return 0, errReadFailed
-		}
-	}
-	return len(b), nil
-}
-
-// int getentropy(void *buf, size_t buflen);
-//export getentropy
-func libc_getentropy(buf unsafe.Pointer, buflen int) int