P3456 R0 system_scheduler on Win32, Darwin and Linux

[wg21bot]

P3456R0 system_scheduler on Win32, Darwin and Linux (Gor Nishanov)

[JeffGarland]

Removing LWG for the moment since SG1 will and then LEWG would be up first.

[inbal2l]

Should be looked at alongside "P2079: System execution context"

[ogiroux]

SG1 does not want to proceed with these changes.

[inbal2l]

SG1 did not recommend applying on "P2079: System execution context", closing for now.
(might be re-considered if additional information comes up and a new revision is published)