driver: force merge objects when building dynamic objects

This patch forces the driver to always merge objects when building
dynamic objects even when ar -L is supported. It is an oversight of
!8887: original rationale of that patch is favoring the relatively
cheap ar -L operation over object merging when ar -L is supported,
which makes sense but only if we are building static objects! Omitting
check for whether we are building dynamic objects will result in
broken .so files with undefined reference errors at executable link
time when building GHC with llvm-ar. Fixes #22210.