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/asm-x86/asm/sigcontext.h b/libc/kernel/uapi/asm-x86/asm/sigcontext.h
index 7d5b4d4..c790950 100644
--- a/libc/kernel/uapi/asm-x86/asm/sigcontext.h
+++ b/libc/kernel/uapi/asm-x86/asm/sigcontext.h
@@ -180,7 +180,7 @@
   __u32 eflags;
   __u32 esp_at_signal;
   __u16 ss, __ssh;
-  struct _fpstate __user * fpstate;
+  struct _fpstate  * fpstate;
   __u32 oldmask;
   __u32 cr2;
 };
@@ -215,7 +215,7 @@
   __u64 trapno;
   __u64 oldmask;
   __u64 cr2;
-  struct _fpstate __user * fpstate;
+  struct _fpstate  * fpstate;
 #ifdef __ILP32__
   __u32 __fpstate_pad;
 #endif