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/input.h b/libc/kernel/uapi/linux/input.h
index 4858c81..1df1398 100644
--- a/libc/kernel/uapi/linux/input.h
+++ b/libc/kernel/uapi/linux/input.h
@@ -173,7 +173,7 @@
   __u16 phase;
   struct ff_envelope envelope;
   __u32 custom_len;
-  __s16 __user * custom_data;
+  __s16  * custom_data;
 };
 struct ff_rumble_effect {
   __u16 strong_magnitude;