[WebAssembly] Use the default alignment for SIMD types.

[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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@245494 91177308-0d34-0410-b5e6-96231b3b80d8