Windows compatibility: Conditionally toggles Process#spawn

[gmenel]

When launching processes on Windows, I received a failure, ArgumentError: wrong exec option symbol: pgroup
Upon inspection of Process#spawn, I noticed that depending on the OS you either pass pgroup: or new_pgroup:. This changes toggles based on Gem.win_platform? which should be stable as it's part of core.

Types of Changes

• Bug fix.

Contribution

• [x] I tested my changes locally.
• [x] I agree to the Developer's Certificate of Origin 1.1.

Thanks for your time.