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/i915_drm.h b/libc/kernel/uapi/drm/i915_drm.h
index a8982bf..794e784 100644
--- a/libc/kernel/uapi/drm/i915_drm.h
+++ b/libc/kernel/uapi/drm/i915_drm.h
@@ -287,18 +287,18 @@
   int DR1;
   int DR4;
   int num_cliprects;
-  struct drm_clip_rect __user * cliprects;
+  struct drm_clip_rect  * cliprects;
 } drm_i915_batchbuffer_t;
 typedef struct _drm_i915_cmdbuffer {
-  char __user * buf;
+  char  * buf;
   int sz;
   int DR1;
   int DR4;
   int num_cliprects;
-  struct drm_clip_rect __user * cliprects;
+  struct drm_clip_rect  * cliprects;
 } drm_i915_cmdbuffer_t;
 typedef struct drm_i915_irq_emit {
-  int __user * irq_seq;
+  int  * irq_seq;
 } drm_i915_irq_emit_t;
 typedef struct drm_i915_irq_wait {
   int irq_seq;
@@ -371,7 +371,7 @@
 #define I915_PARAM_OA_TIMESTAMP_FREQUENCY 57
 struct drm_i915_getparam {
   __s32 param;
-  int __user * value;
+  int  * value;
 };
 typedef struct drm_i915_getparam drm_i915_getparam_t;
 #define I915_SETPARAM_USE_MI_BATCHBUFFER_START 1
@@ -387,7 +387,7 @@
   int region;
   int alignment;
   int size;
-  int __user * region_offset;
+  int  * region_offset;
 } drm_i915_mem_alloc_t;
 typedef struct drm_i915_mem_free {
   int region;