Initial introduction of zfinx (#75)

* Adds Zfinx enable flag

* Hardwire misa.{f,d} and mstats.FS to 0

* Moving nan boxing functions to fdext_reg

* Swaps register names for floating point instructions

Adds new mapping to swap register names, and use it in all assembly clauses

* Disable Floating point loads, stores and moves

* Add X_or_F_s and X_or_F_d functions, and use them to access all registers for floating points

Changes register accessed for floating point instructions and modify nan boxing functions for zfinx

* Formatting

Remove couple of misplaced whitespace, unnecessary parens

* Fix inconsistent indentation in insts_dext file

* Fix spacing in fdext_regs

* Remove redundant comparasion with true/ false

* Constistant tuples spacing and removes couple of unnecessary parens.

* Consistent functions declaration & calls spacing and removes couple of unnecessary parens.

* Consistent spacing and removes couple of unnecessary comparasion with true/false

* Make spacing consistent

* Remove checks from execution stage

* Add checks to encdec stage