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/i2c-dev.h b/libc/kernel/uapi/linux/i2c-dev.h
index 07fdda2..7a25e20 100644
--- a/libc/kernel/uapi/linux/i2c-dev.h
+++ b/libc/kernel/uapi/linux/i2c-dev.h
@@ -33,10 +33,10 @@
   __u8 read_write;
   __u8 command;
   __u32 size;
-  union i2c_smbus_data __user * data;
+  union i2c_smbus_data  * data;
 };
 struct i2c_rdwr_ioctl_data {
-  struct i2c_msg __user * msgs;
+  struct i2c_msg  * msgs;
   __u32 nmsgs;
 };
 #define I2C_RDWR_IOCTL_MAX_MSGS 42