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/usbdevice_fs.h b/libc/kernel/uapi/linux/usbdevice_fs.h
index 5fef522..af8681b 100644
--- a/libc/kernel/uapi/linux/usbdevice_fs.h
+++ b/libc/kernel/uapi/linux/usbdevice_fs.h
@@ -27,13 +27,13 @@
   __u16 wIndex;
   __u16 wLength;
   __u32 timeout;
-  void __user * data;
+  void  * data;
 };
 struct usbdevfs_bulktransfer {
   unsigned int ep;
   unsigned int len;
   unsigned int timeout;
-  void __user * data;
+  void  * data;
 };
 struct usbdevfs_setinterface {
   unsigned int interface;
@@ -41,7 +41,7 @@
 };
 struct usbdevfs_disconnectsignal {
   unsigned int signr;
-  void __user * context;
+  void  * context;
 };
 #define USBDEVFS_MAXDRIVERNAME 255
 struct usbdevfs_getdriver {
@@ -80,7 +80,7 @@
   unsigned char endpoint;
   int status;
   unsigned int flags;
-  void __user * buffer;
+  void  * buffer;
   int buffer_length;
   int actual_length;
   int start_frame;
@@ -90,13 +90,13 @@
   };
   int error_count;
   unsigned int signr;
-  void __user * usercontext;
+  void  * usercontext;
   struct usbdevfs_iso_packet_desc iso_frame_desc[];
 };
 struct usbdevfs_ioctl {
   int ifno;
   int ioctl_code;
-  void __user * data;
+  void  * data;
 };
 struct usbdevfs_hub_portinfo {
   char nports;