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/atmdev.h b/libc/kernel/uapi/linux/atmdev.h
index 69eb240..74cdfc1 100644
--- a/libc/kernel/uapi/linux/atmdev.h
+++ b/libc/kernel/uapi/linux/atmdev.h
@@ -86,7 +86,7 @@
 #define ATM_LM_RMT_ANALOG __ATM_LM_MKRMT(__ATM_LM_ANALOG)
 struct atm_iobuf {
   int length;
-  void __user * buffer;
+  void  * buffer;
 };
 #define ATM_CI_MAX - 1
 struct atm_cirange {