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/drm/r128_drm.h b/libc/kernel/uapi/drm/r128_drm.h
index 618b6dc..3e013b1 100644
--- a/libc/kernel/uapi/drm/r128_drm.h
+++ b/libc/kernel/uapi/drm/r128_drm.h
@@ -204,13 +204,13 @@
     R128_READ_PIXELS = 0x04
   } func;
   int n;
-  int __user * x;
-  int __user * y;
-  unsigned int __user * buffer;
-  unsigned char __user * mask;
+  int  * x;
+  int  * y;
+  unsigned int  * buffer;
+  unsigned char  * mask;
 } drm_r128_depth_t;
 typedef struct drm_r128_stipple {
-  unsigned int __user * mask;
+  unsigned int  * mask;
 } drm_r128_stipple_t;
 typedef struct drm_r128_indirect {
   int idx;
@@ -227,7 +227,7 @@
 #define R128_PARAM_IRQ_NR 1
 typedef struct drm_r128_getparam {
   int param;
-  void __user * value;
+  void  * value;
 } drm_r128_getparam_t;
 #ifdef __cplusplus
 }