rts/rts.cabal: Fix build on Illumos

Illumos's linker dislikes undefined symbols remaining in executables, just like Darwin's one. Pass a flag -z nodefs to suppress errors. The flag is equivalent to -undefined dynamic_lookup on Darwin.

• if your MR may break existing programs (e.g. touches base or causes the compiler to reject programs), please describe the expected breakage and add the user-facing label. This will run ghc/head.hackage> to characterise the effect of your change on Hackage.
• ensure that your commits are either individually buildable or squashed
• ensure that your commit messages describe what they do (referring to tickets using #NNNN syntax when appropriate)
• have added source comments describing your change. For larger changes you likely should add a [Note][notes] and cross-reference it from the relevant places.
• add a [testcase to the testsuite][adding test].
• updates the users guide if applicable
• mentions new features in the release notes for the next release