Merge remote-tracking branch 'remotes/stsquad/tags/pull-softfloat-refactor-210218...

This is the re-factor of softfloat:

  - shared common code path float16/32/64
  - well commented and easy to follow code
  - added a bunch of float16 support

While some operations are slower the key ones exercised by the
floating point dbt-bench are the same: https://i.imgur.com/oXNJNql.png

# gpg: Signature made Wed 21 Feb 2018 10:44:14 GMT
# gpg:                using RSA key FBD0DB095A9E2A44
# gpg: Good signature from "Alex Bennée (Master Work Key) <alex.bennee@linaro.org>"
# Primary key fingerprint: 6685 AE99 E751 67BC AFC8  DF35 FBD0 DB09 5A9E 2A44

* remotes/stsquad/tags/pull-softfloat-refactor-210218-1: (22 commits)
  fpu/softfloat: re-factor sqrt
  fpu/softfloat: re-factor compare
  fpu/softfloat: re-factor minmax
  fpu/softfloat: re-factor scalbn
  fpu/softfloat: re-factor int/uint to float
  fpu/softfloat: re-factor float to int/uint
  fpu/softfloat: re-factor round_to_int
  fpu/softfloat: re-factor muladd
  fpu/softfloat: re-factor div
  fpu/softfloat: re-factor mul
  fpu/softfloat: re-factor add/sub
  fpu/softfloat: define decompose structures
  fpu/softfloat: move the extract functions to the top of the file
  fpu/softfloat: improve comments on ARM NaN propagation
  include/fpu/softfloat: add some float16 constants
  include/fpu/softfloat: implement float16_set_sign helper
  include/fpu/softfloat: implement float16_chs helper
  include/fpu/softfloat: implement float16_abs helper
  target/*/cpu.h: remove softfloat.h
  fpu/softfloat-types: new header to prevent excessive re-builds
  ...

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>