gc:orc defines gcArc

[Clyybber]

#15811 (comment)
#17342 (comment)

[timotheecour]

• while you're at it, we should add a defined gcRefc for gc:refc
• this is a breaking change [1]; instead we should introduce gcArcLike instead, which wouldn't be a breaking change and is probably a bit easier to use (avoids things like when defined(gcArc) and not defined(gcOrc), which also isn't future prof in case another arc variant is later introduced)

when defined(gcArc): echo 1
elif defined(gcOrc): echo 2
else: echo 3

before PR:
nim r --gc:orc main # 2
after PR:
nim r --gc:orc main # 1

(gcArcLike is better than gcArcOrc in case we later add another variant than orc based on arc, but we can bikeshed the name, eg gcSomeArc etc)

gcArcLike abides by the rule that each gc shall have its corresponding exclusive defined symbol

[Araq]

We have gcDestructors for ARC-like GCs already. :-)