[WebAssembly] Add simd128-unimplemented subtarget feature

This is a second attempt at r350778, which was reverted in
r350789. The only change is that the unimplemented-simd128 feature has
been renamed simd128-unimplemented, since naming it
unimplemented-simd128 somehow made the simd128 feature flag enable the
unimplemented-simd128 feature on Windows.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@350791 91177308-0d34-0410-b5e6-96231b3b80d8
21 files changed