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/if_tunnel.h b/libc/kernel/uapi/linux/if_tunnel.h
index 01c2dc9..9b85568 100644
--- a/libc/kernel/uapi/linux/if_tunnel.h
+++ b/libc/kernel/uapi/linux/if_tunnel.h
@@ -142,7 +142,7 @@
   __IFLA_GRE_MAX,
 };
 #define IFLA_GRE_MAX (__IFLA_GRE_MAX - 1)
-#define VTI_ISVTI ((__force __be16) 0x0001)
+#define VTI_ISVTI (( __be16) 0x0001)
 enum {
   IFLA_VTI_UNSPEC,
   IFLA_VTI_LINK,