[llvm-config] Print --system-libs only when static linking

Modify the --system-libs option in llvm-config to print system libs only
when using static linking. The system libraries are irrelevant when
linking to a shared library since the library has appropriate library
dependencies embedded.

Modify the --system-libs test appropriately to force static linking, and
disable it if static libs are not available (i.e. BUILD_SHARED_LIBS is
enabled).

Differential Revision: https://reviews.llvm.org/D27805

llvm-svn: 291285