internal/syscall/unix: implement GetRandom for WASI targets

Go 1.26's crypto/internal/sysrand uses internal/syscall/unix.GetRandom
on Linux-like targets (including wasip2 which sets GOOS=linux). The
existing stub panicked with 'todo: unix.GetRandom', causing crypto/ecdsa
tests to fail on wasip2.

Implement GetRandom on WASI targets (wasip1, wasip2) by calling the
arc4random_buf libc function that TinyGo's runtime already provides.
For other TinyGo targets, return ENOSYS so sysrand can fall back to
/dev/urandom.

src/internal/syscall/unix/getrandom.go

@@ -1,5 +1,9 @@
+//go:build !tinygo.wasm
+
 package unix
 
+import "syscall"
+
 type GetRandomFlag uintptr
 
 const (
@@ -8,5 +12,7 @@ const (
 )
 
 func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
-	panic("todo: unix.GetRandom")
+	// Not supported on most TinyGo targets.
+	// On real Linux the sysrand package will fall back to /dev/urandom.
+	return 0, syscall.ENOSYS
 }

src/internal/syscall/unix/getrandom_wasi.go

@@ -0,0 +1,25 @@
+//go:build tinygo.wasm
+
+package unix
+
+import "unsafe"
+
+type GetRandomFlag uintptr
+
+const (
+	GRND_NONBLOCK GetRandomFlag = 0x0001
+	GRND_RANDOM   GetRandomFlag = 0x0002
+)
+
+func GetRandom(p []byte, flags GetRandomFlag) (n int, err error) {
+	if len(p) == 0 {
+		return 0, nil
+	}
+	libc_arc4random_buf(unsafe.Pointer(&p[0]), uint(len(p)))
+	return len(p), nil
+}
+
+// void arc4random_buf(void *buf, size_t buflen);
+//
+//export arc4random_buf
+func libc_arc4random_buf(buf unsafe.Pointer, buflen uint)