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/kvm.h b/libc/kernel/uapi/linux/kvm.h
index 9566f01..9d33399 100644
--- a/libc/kernel/uapi/linux/kvm.h
+++ b/libc/kernel/uapi/linux/kvm.h
@@ -457,7 +457,7 @@
   __u32 slot;
   __u32 padding1;
   union {
-    void __user * dirty_bitmap;
+    void  * dirty_bitmap;
     __u64 padding2;
   };
 };
@@ -466,7 +466,7 @@
   __u32 num_pages;
   __u64 first_page;
   union {
-    void __user * dirty_bitmap;
+    void  * dirty_bitmap;
     __u64 padding2;
   };
 };