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/sem.h b/libc/kernel/uapi/linux/sem.h
index 98ed1bf..c2f25e7 100644
--- a/libc/kernel/uapi/linux/sem.h
+++ b/libc/kernel/uapi/linux/sem.h
@@ -48,10 +48,10 @@
 };
 union __kernel_legacy_semun {
   int val;
-  struct __kernel_legacy_semid_ds __user * buf;
-  unsigned short __user * array;
-  struct seminfo __user * __buf;
-  void __user * __pad;
+  struct __kernel_legacy_semid_ds  * buf;
+  unsigned short  * array;
+  struct seminfo  * __buf;
+  void  * __pad;
 };
 struct seminfo {
   int semmap;