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/vfio.h b/libc/kernel/uapi/linux/vfio.h
index 7dc3440..3c1821c 100644
--- a/libc/kernel/uapi/linux/vfio.h
+++ b/libc/kernel/uapi/linux/vfio.h
@@ -390,7 +390,7 @@
 struct vfio_bitmap {
   __u64 pgsize;
   __u64 size;
-  __u64 __user * data;
+  __u64  * data;
 };
 struct vfio_iommu_type1_dma_unmap {
   __u32 argsz;