driver: use interpreterDynamic predicate in preloadLib

This commit use the interpreterDynamic predicate in preloadLib to
decide if we should do dynLoadObjs instead of loadObj. Previously we
used hostIsDynamic which was only written with non-cross internal
interpreter in mind.

The testsuite is also adjusted to remove hard-wired -fPIC flag for
cbits (doesn't work in i386 RTS linker in vanilla way, #25260) and
properly pass ghc_th_way_flags to ghc.