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/chio.h b/libc/kernel/uapi/linux/chio.h
index 725e760..74f286f 100644
--- a/libc/kernel/uapi/linux/chio.h
+++ b/libc/kernel/uapi/linux/chio.h
@@ -71,7 +71,7 @@
 #define CP_INVERT 1
 struct changer_element_status {
   int ces_type;
-  unsigned char __user * ces_data;
+  unsigned char  * ces_data;
 };
 #define CESTATUS_FULL 0x01
 #define CESTATUS_IMPEXP 0x02