[lld-macho][NFC] Refactor syslibroot / library path lookup

* Move computation of systemLibraryRoots into a separate function, so we
  can add more functionality to it without things becoming unwieldy
* Have `getSearchPaths` and related functions return by value instead of
  by output parameter. NRVO should ensure that performance is unaffected.

Reviewed By: #lld-macho, smeenai

Differential Revision: https://reviews.llvm.org/D87959