[cmake] Add LLVM_UNITTEST_LINK_FLAGS option

Add an option to specify additional linker flags for unit tests only.
For example, this allows using something like
-DLLVM_UNITTEST_LINK_FLAGS="-Wl,-plugin-opt=O0" if you're doing LTO
builds, or -DLLVM_UNITTEST_LINK_FLAGS="-fno-lto" if you're using
fat LTO objects.

The build system already does this itself if the LLVM_ENABLE_LTO
flag is used, but this does not cover all possible LTO configurations.

Differential Revision: https://reviews.llvm.org/D154212