[kernel] Allow blocking on empty channels

This commit adds a new flag, j6_channel_block, and a new flags param to
the channel_receive syscall. When the block flag is specified, the
caller will block waiting for data on the channel if the channel is
empty.

src/libraries/j6/j6/flags.h

@@ -9,6 +9,10 @@ enum j6_vm_flags {
     j6_vm_flag_MAX
 };
 
+enum j6_channel_flags {
+    j6_channel_block = 0x01,
+};
+
 enum j6_mailbox_flags {
     j6_mailbox_block = 0x01,
 };