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/kd.h b/libc/kernel/uapi/linux/kd.h
index 2385037..903681b 100644
--- a/libc/kernel/uapi/linux/kd.h
+++ b/libc/kernel/uapi/linux/kd.h
@@ -27,7 +27,7 @@
 struct consolefontdesc {
   unsigned short charcount;
   unsigned short charheight;
-  char __user * chardata;
+  char  * chardata;
 };
 #define PIO_FONTRESET 0x4B6D
 #define GIO_CMAP 0x4B70
@@ -68,7 +68,7 @@
 };
 struct unimapdesc {
   unsigned short entry_ct;
-  struct unipair __user * entries;
+  struct unipair  * entries;
 };
 #define PIO_UNIMAP 0x4B67
 #define PIO_UNIMAPCLR 0x4B68
@@ -147,7 +147,7 @@
   unsigned int flags;
   unsigned int width, height;
   unsigned int charcount;
-  unsigned char __user * data;
+  unsigned char  * data;
 };
 struct console_font {
   unsigned int width, height;