Remove ARM/MIPS fenv duplication.

Bug: http://b/32709207
Test: ran tests
Change-Id: I58d354e9f8b1e6023427ba271bf39e4e24b7f8ab
diff --git a/libc/include/bits/fenv_x86.h b/libc/include/bits/fenv_x86.h
index 75b109b..3381cea 100644
--- a/libc/include/bits/fenv_x86.h
+++ b/libc/include/bits/fenv_x86.h
@@ -26,8 +26,7 @@
  * $FreeBSD: src/lib/msun/i387/fenv.h,v 1.4 2005/03/17 22:21:46 das Exp $
  */
 
-#ifndef _BITS_FENV_X86_H_
-#define _BITS_FENV_X86_H_
+#pragma once
 
 #include <sys/types.h>
 
@@ -65,5 +64,3 @@
 #define FE_TOWARDZERO 0x0c00
 
 __END_DECLS
-
-#endif