fpu: Implement floating-point context switching

* Employ a simplistic approach in which the state is always saved and
  restored once a task uses floating point.  Lazy restore should be a
  straightfoward extension, along with a counter mechanism for reverting
  to eager behavior after a succession of traps exceeds some threshold.
* Introduce architecture-specific <asm/switch_to.h> header.
* Compile with the -msoft-float option to induce linker errors upon
  undesired floating point usage in kernel code.
* Note: This initial implementation is not preemption-safe.