[CMake] Fix generation of exported targets in build directory

CMake generates the exports file wrongly if the CMAKE_BUILD_TYPE is
unset for multi-configuration generators.

The generated file lib/cmake/polly/PollyExports-all.cmake then contains:

set_target_properties(LLVMPolly PROPERTIES
              IMPORTED_LOCATION_ my_build_dir/lib/LLVMPolly.so)
set_target_properties(Polly PROPERTIES
              IMPORTED_LOCATION_ my_build_dir/lib/libPolly.a)

This patch conditionalizes the underscore.

Patch by: Marcin Copik

Differential Revision: D53376