5261 libm should stop using synonyms.h

*** 25,35 ****
  /*
   * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
! #pragma weak casinh = __casinh
  
  /* INDENT OFF */
  /*
   * dcomplex casinh(dcomplex z);
   * casinh z = -i casin iz .
--- 25,35 ----
  /*
   * Copyright 2006 Sun Microsystems, Inc.  All rights reserved.
   * Use is subject to license terms.
   */
  
! #pragma weak __casinh = casinh
  
  /* INDENT OFF */
  /*
   * dcomplex casinh(dcomplex z);
   * casinh z = -i casin iz .