Remove some fpu/fctrl test cases from coverage statistics

Created by: infinitymdm

This PR turns coverage off for the following instructions which are not reached when running ./regression-wally -coverage:

• Conversions to/from half precision
  • fcvt.h.(s/d/q)
  • fcvt.w.h
  • fcvt.wu.h
  • fcvt.l.h
  • fcvt.lu.h
  • fcvt.h.w
  • fcvt.h.wu
  • fcvt.h.l
  • fcvt.h.lu
• Conversions to/from quad precision
  • fcvt.q.(s/h/d)
  • fcvt.q.w
  • fcvt.q.wu
  • fcvt.q.l
  • fcvt.q.lu
  • fcvt.w.q
  • fcvt.wu.q
  • fcvt.l.q
  • fcvt.lu.q

These instructions are instead tested by running fpu.s with the run-elf.bash script.

Not 100% certain this is the correct approach here. Feel free to point me in the right direction!