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/media.h b/libc/kernel/uapi/linux/media.h
index 869d47b..fe97e90 100644
--- a/libc/kernel/uapi/linux/media.h
+++ b/libc/kernel/uapi/linux/media.h
@@ -125,8 +125,8 @@
 };
 struct media_links_enum {
   __u32 entity;
-  struct media_pad_desc __user * pads;
-  struct media_link_desc __user * links;
+  struct media_pad_desc  * pads;
+  struct media_link_desc  * links;
   __u32 reserved[4];
 };
 #define MEDIA_INTF_T_DVB_BASE 0x00000100