[WebAssembly] Use the default alignment for SIMD types.

Previously WebAssembly's datalayout string had -v128:8:128. This had been an
attempt to declare a certain level of support for unaligned SIMD accesses.
However, clang makes its own determinations for SIMD alignment that are
independent of the datalayout string, so this wasn't actually meaningful.

llvm-svn: 245494