Update compile target dependency.

test/elf/Mips/hilo16-*.test depends on llvm-mc, so we need to
make CMake to build that before running the tests.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@220768 91177308-0d34-0410-b5e6-96231b3b80d8