Remove ARM/MIPS fenv duplication.

Bug: http://b/32709207
Test: ran tests
Change-Id: I58d354e9f8b1e6023427ba271bf39e4e24b7f8ab
diff --git a/libc/include/bits/fenv_x86_64.h b/libc/include/bits/fenv_x86_64.h
index 006d19e..ddb6a5c 100644
--- a/libc/include/bits/fenv_x86_64.h
+++ b/libc/include/bits/fenv_x86_64.h
@@ -24,8 +24,7 @@
  * SUCH DAMAGE.
  */
 
-#ifndef _BITS_FENV_X86_64_H_
-#define _BITS_FENV_X86_64_H_
+#pragma once
 
 #include <sys/types.h>
 
@@ -91,5 +90,3 @@
 typedef __uint32_t fexcept_t;
 
 __END_DECLS
-
-#endif