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/savage_drm.h b/libc/kernel/uapi/drm/savage_drm.h
index ae87d21..efc0ae6 100644
--- a/libc/kernel/uapi/drm/savage_drm.h
+++ b/libc/kernel/uapi/drm/savage_drm.h
@@ -68,14 +68,14 @@
 } drm_savage_init_t;
 typedef union drm_savage_cmd_header drm_savage_cmd_header_t;
 typedef struct drm_savage_cmdbuf {
-  drm_savage_cmd_header_t __user * cmd_addr;
+  drm_savage_cmd_header_t  * cmd_addr;
   unsigned int size;
   unsigned int dma_idx;
   int discard;
-  unsigned int __user * vb_addr;
+  unsigned int  * vb_addr;
   unsigned int vb_size;
   unsigned int vb_stride;
-  struct drm_clip_rect __user * box_addr;
+  struct drm_clip_rect  * box_addr;
   unsigned int nbox;
 } drm_savage_cmdbuf_t;
 #define SAVAGE_WAIT_2D 0x1