Fix FP traps on OSX/x86-64.

Fix FP traps on OSX/x86-64.

  * De-cripple SB-INT:SET-FLOATING-POINT-MODES for this platform.

  * Enable restoring the FPU control word during interrupt handling
on this platform (RESTORE_FP_CONTROL_FROM_CONTEXT).

  * Implement restoring the FPU control word on this platform
(os_restore_fp_control).

  * Update :FAILS-ON information for the now-passing tests.

  * Insert an addtional copy of test float.pure.lisp /
(ADDITION-OVERFLOW BUG-372) to detect failure to restore the FPU
control word in signal handling, with appropriate commentary.