P3008 R5 Atomic floating-point min/max

[wg21bot]

P3008R0 Atomic floating-point min/max (Gonzalo Brito Gadeschi)

[cor3ntin]

@ogiroux , @mattkretz - can SG1 and SG6 please review and tag LEWG once forwarded

[ogiroux]

SG1 poll in Kona:

DIRECTION POLL:
1. The default fetch_min(T, mo=SC)->T, fetch_max(T, mo=SC)->T should exist
2. The default fetch_min, fetch_max should have these semantics:
  * Which value is stored in the atomic is unspecified if an input is a NaN
  * Implementations should treat -0 as less than +0 (use normative encouragement)
  * (Note: must _not_ require std::min/max)
3. Users need a way to specify semantics (up to LEWG how to achieve this):
  * Must-have: fminimum/fmaximum
  * Must-have: fminimum_num/fmaximum_num
  * Can have if LEWG desires it but we do not: std::min/max
SF F N A SA
 5 6 2 0 0
Unanimous consent

[inbal2l]

Needs to be rebased on P0493. (Resolves issues, see #866)