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/ppp-ioctl.h b/libc/kernel/uapi/linux/ppp-ioctl.h
index 69c6e6d..165d447 100644
--- a/libc/kernel/uapi/linux/ppp-ioctl.h
+++ b/libc/kernel/uapi/linux/ppp-ioctl.h
@@ -56,7 +56,7 @@
   enum NPmode mode;
 };
 struct ppp_option_data {
-  __u8 __user * ptr;
+  __u8  * ptr;
   __u32 length;
   int transmit;
 };