Clarify and test the interaction of -with-rtsopts and -rtsopts flags

Looking at the user manual it's not clear how do -with-rtsopts and -rtsopts flags interact. I'd expect -with-rtsopts to work even with -rtsopts=ignoreAll, but I don't know if this is really the case, and because we don't have a way to print RTS flags (see #15261 (closed) for this) it's hard to make sure this works as expected.

Trac metadata

Trac field	Value
Version	8.5
Type	Task
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	Compiler
Test case
Differential revisions
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Blocking
CC	AndreasK
Operating system
Architecture