Rename include/fenv.h to <openlibm_fenv.h>.

OpenLibm has an implementation of fenv.h internally. This may be
problematic in case you want it to build against the host system's
implementation, as it would require you to somehow take the fenv.h file
out of the compiler search path.

Simply use a different naming scheme, similar to openlibm.h and
openlibm_complex.h. If we want to build against the host's fenv.h, we
can simply add an '#include <fenv.h>' from within this header.

include/fenv.h

@@ -1,9 +0,0 @@
-#ifdef __arm__
-#include "../arm/fenv.h"
-#else
-#ifdef __LP64
-#include "../amd64/fenv.h"
-#else
-#include "../i387/fenv.h"
-#endif
-#endif