Merge remote-tracking branch 'remotes/rth-gitlab/tags/pull-fp-20210516' into staging

Reorg FloatParts to use QEMU_GENERIC.
Begin replacing the Berkeley float128 routines with FloatParts128.
  - includes a new implementation of float128_muladd
  - includes the snan silencing that was missing from
    float{32,64}_to_float128 and float128_to_float{32,64}.
  - does not include float128_min/max* (written but not yet reviewed).

# gpg: Signature made Sun 16 May 2021 13:27:10 BST
# gpg:                using RSA key 7A481E78868B4DB6A85A05C064DF38E8AF7E215F
# gpg:                issuer "richard.henderson@linaro.org"
# gpg: Good signature from "Richard Henderson <richard.henderson@linaro.org>" [full]
# Primary key fingerprint: 7A48 1E78 868B 4DB6 A85A  05C0 64DF 38E8 AF7E 215F

* remotes/rth-gitlab/tags/pull-fp-20210516: (46 commits)
  softfloat: Move round_to_int_and_pack to softfloat-parts.c.inc
  softfloat: Move round_to_int to softfloat-parts.c.inc
  softfloat: Convert float-to-float conversions with float128
  softfloat: Split float_to_float
  softfloat: Move div_floats to softfloat-parts.c.inc
  softfloat: Introduce sh[lr]_double primitives
  softfloat: Tidy mul128By64To192
  softfloat: Use add192 in mul128To256
  softfloat: Use mulu64 for mul64To128
  softfloat: Move muladd_floats to softfloat-parts.c.inc
  softfloat: Move mul_floats to softfloat-parts.c.inc
  softfloat: Implement float128_add/sub via parts
  softfloat: Move addsub_floats to softfloat-parts.c.inc
  softfloat: Use uadd64_carry, usub64_borrow in softfloat-macros.h
  softfloat: Move round_canonical to softfloat-parts.c.inc
  softfloat: Move sf_canonicalize to softfloat-parts.c.inc
  softfloat: Move pick_nan_muladd to softfloat-parts.c.inc
  softfloat: Move pick_nan to softfloat-parts.c.inc
  softfloat: Move return_nan to softfloat-parts.c.inc
  softfloat: Convert float128_default_nan to parts
  ...

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