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/cdrom.h b/libc/kernel/uapi/linux/cdrom.h
index 0fe0727..bd1d7df 100644
--- a/libc/kernel/uapi/linux/cdrom.h
+++ b/libc/kernel/uapi/linux/cdrom.h
@@ -128,7 +128,7 @@
   union cdrom_addr addr;
   __u8 addr_format;
   int nframes;
-  __u8 __user * buf;
+  __u8  * buf;
 };
 struct cdrom_multisession {
   union cdrom_addr addr;
@@ -149,16 +149,16 @@
 #define CGC_DATA_NONE 3
 struct cdrom_generic_command {
   unsigned char cmd[CDROM_PACKET_SIZE];
-  unsigned char __user * buffer;
+  unsigned char  * buffer;
   unsigned int buflen;
   int stat;
-  struct request_sense __user * sense;
+  struct request_sense  * sense;
   unsigned char data_direction;
   int quiet;
   int timeout;
   union {
-    void __user * reserved[1];
-    void __user * unused;
+    void  * reserved[1];
+    void  * unused;
   };
 };
 struct cdrom_timed_media_change_info {