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/ipc.h b/libc/kernel/uapi/linux/ipc.h
index a0699cf..4ca9ee1 100644
--- a/libc/kernel/uapi/linux/ipc.h
+++ b/libc/kernel/uapi/linux/ipc.h
@@ -42,7 +42,7 @@
 #define IPC_OLD 0
 #define IPC_64 0x0100
 struct ipc_kludge {
-  struct msgbuf __user * msgp;
+  struct msgbuf  * msgp;
   long msgtyp;
 };
 #define SEMOP 1