Accept GHCRTS=-N1 when not compiled with -threaded

current behaviour:

> ghc Main.hs -o test
> ./test +RTS -N1
test: the flag -N1 requires the program to be built with -threaded
[blah blah]

requested:

> ghc Main.hs -o test
> ./test +RTS -N1
hello world

Motivation: In certain environments, threaded is bad idea due to resource constraints even when multiple capabilities are available, because resident memory can increase with parallel execution. Slapping GHCRTS=-N1 onto those environments would be nice, but breaks for things not compiled with -threaded.

Trac metadata

Trac field	Value
Version	8.6.1
Type	FeatureRequest
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Runtime System
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