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/keyctl.h b/libc/kernel/uapi/linux/keyctl.h
index 01ea576..b534f5e 100644
--- a/libc/kernel/uapi/linux/keyctl.h
+++ b/libc/kernel/uapi/linux/keyctl.h
@@ -80,8 +80,8 @@
   __s32 base;
 };
 struct keyctl_kdf_params {
-  char __user * hashname;
-  char __user * otherinfo;
+  char  * hashname;
+  char  * otherinfo;
   __u32 otherinfolen;
   __u32 __spare[8];
 };