[X86] Fix -DBUILD_SHARED_LIBS=on builds after D69568/llvmorg-10-init-8877-g3a399c09878

In -DBUILD_SHARED_LIBS=on builds, a component must specify its direct dependencies to satisfy -Wl,-z,defs (added by llvm/modules/HandleLLVMOptions.cmake).

Core is a direct dependency via transitive header inclusion:

  ld.lld: error: undefined symbol: llvm::LLVMContext::LLVMContext()
  >>> referenced by MachineSizeOptsTest.cpp
  >>>               unittests/Target/X86/CMakeFiles/X86Tests.dir/MachineSizeOptsTest.cpp.o:(testing::internal::TestFactoryImpl<(anonymous namespace)::MachineSizeOptsTest_Test_Test>::CreateTest())

MC is a direct dependency via transitive header inclusion:

  ld.lld: error: undefined symbol: llvm::MCTargetOptions::MCTargetOptions()
  >>> referenced by MachineSizeOptsTest.cpp
  >>>               unittests/Target/X86/CMakeFiles/X86Tests.dir/MachineSizeOptsTest.cpp.o:((anonymous namespace)::MachineSizeOptsTest::SetUp())