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/atm.h b/libc/kernel/uapi/linux/atm.h
index 488fa05..72d329d 100644
--- a/libc/kernel/uapi/linux/atm.h
+++ b/libc/kernel/uapi/linux/atm.h
@@ -140,7 +140,7 @@
 struct atmif_sioc {
   int number;
   int length;
-  void __user * arg;
+  void  * arg;
 };
 typedef unsigned short atm_backend_t;
 #endif