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/coda.h b/libc/kernel/uapi/linux/coda.h
index aad1b97..5795cdf 100644
--- a/libc/kernel/uapi/linux/coda.h
+++ b/libc/kernel/uapi/linux/coda.h
@@ -505,13 +505,13 @@
 };
 #define PIOCPARM_MASK 0x0000ffff
 struct ViceIoctl {
-  void __user * in;
-  void __user * out;
+  void  * in;
+  void  * out;
   u_short in_size;
   u_short out_size;
 };
 struct PioctlData {
-  const char __user * path;
+  const char  * path;
   int follow;
   struct ViceIoctl vi;
 };