Remove unneeded tests for <sys/cdefs.h>.

This test is also present in FreeBSD's <machine/ieeefp.h>. For FreeBSD
it makes sense, but for a portable math library, we cannot assume that
the system has a header file like <sys/cdefs.h> and that it uses a
common header guard.

amd64/bsd_ieeefp.h

@@ -38,10 +38,6 @@
 #ifndef _MACHINE_IEEEFP_H_
 #define _MACHINE_IEEEFP_H_
 
-#if !defined(_SYS_CDEFS_H) && !defined(_SYS_CDEFS_H_) && !defined(_CDEFS_H_)
-#error this file needs sys/cdefs.h as a prerequisite
-#endif
-
 /*
  * FP rounding modes
  */