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/comedi.h b/libc/kernel/uapi/linux/comedi.h
index e0d015a..465d1bf 100644
--- a/libc/kernel/uapi/linux/comedi.h
+++ b/libc/kernel/uapi/linux/comedi.h
@@ -244,14 +244,14 @@
 struct comedi_insn {
   unsigned int insn;
   unsigned int n;
-  unsigned int __user * data;
+  unsigned int  * data;
   unsigned int subdev;
   unsigned int chanspec;
   unsigned int unused[3];
 };
 struct comedi_insnlist {
   unsigned int n_insns;
-  struct comedi_insn __user * insns;
+  struct comedi_insn  * insns;
 };
 struct comedi_cmd {
   unsigned int subdev;
@@ -268,19 +268,19 @@
   unsigned int stop_arg;
   unsigned int * chanlist;
   unsigned int chanlist_len;
-  short __user * data;
+  short  * data;
   unsigned int data_len;
 };
 struct comedi_chaninfo {
   unsigned int subdev;
-  unsigned int __user * maxdata_list;
-  unsigned int __user * flaglist;
-  unsigned int __user * rangelist;
+  unsigned int  * maxdata_list;
+  unsigned int  * flaglist;
+  unsigned int  * rangelist;
   unsigned int unused[4];
 };
 struct comedi_rangeinfo {
   unsigned int range_type;
-  void __user * range_ptr;
+  void  * range_ptr;
 };
 struct comedi_krange {
   int min;