uapi headers: simplify the <linux/compiler.h> stuff.

We don't really need <linux/compiler.h> and <linux/compiler_types.h>. We
already have a mechanism to remove unused macros, so let's do that. We
don't currently have a way to remove unused #includes, so we still need
<linux/compiler.h> and <linux/compiler_types.h> files (but I've clarified
the comments in them).

I've kept the empty definitions of `__user` and `__force` for source
compatibility. (We had one security test at least that was assuming
a kernel struct definition will "just work".)

Bug: http://b/262917450
Test: treehugger
Change-Id: Iacbbbc1aeef9a4fac52dabd7811ab875cc267d4f
diff --git a/libc/kernel/uapi/linux/futex.h b/libc/kernel/uapi/linux/futex.h
index c80b90c..8d1a2a4 100644
--- a/libc/kernel/uapi/linux/futex.h
+++ b/libc/kernel/uapi/linux/futex.h
@@ -59,12 +59,12 @@
   __u32 __reserved;
 };
 struct robust_list {
-  struct robust_list __user * next;
+  struct robust_list  * next;
 };
 struct robust_list_head {
   struct robust_list list;
   long futex_offset;
-  struct robust_list __user * list_op_pending;
+  struct robust_list  * list_op_pending;
 };
 #define FUTEX_WAITERS 0x80000000
 #define FUTEX_OWNER_DIED 0x40000000