Move the FPU save/restore stuff from os.h to arch.h

 * src/lisp/os.h
   * Move macros and definitions for FPU save/restore from here.
 * src/lisp/arch.h
   * Put architecture neutral stuff from os.h here.
 * src/lisp/ppc-arch.h
 * src/lisp/sparc-arch.h
 * src/lisp/x86-arch.h
   * Implement the FPU save/restore macros here for each supported
     architecture.