Fix build when no native target is enabled

The JITTests and MCJITTests unit test targets require a native arch with JIT
support, otherwise fail to link.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210411 91177308-0d34-0410-b5e6-96231b3b80d8
Re-enable ARM ELF tests

r210408 inadvertently disabled them when X86 is not selected.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@210412 91177308-0d34-0410-b5e6-96231b3b80d8