[CMake] Don't generate linker script only when shared library isn't statically linked

Since r337668, we support statically linking dependencies only to shared
or static library. However, that change hasn't updated the check whether
to generate a linker script. We shouldn't generate linker script only in
the case when we aren't statically linked ABI into the shared library.

Differential Revision: https://reviews.llvm.org/D49834

llvm-svn: 338006