[WebAssembly] Switch to a more traditional assembly syntax

[WebAssembly] Switch to a more traditional assembly syntax

This new syntax is built around putting each instruction on its own line
in a "mnemonic op, op, op" like syntax. It also uses conventional data
section directives like ".byte" and so on rather than requiring everything
to be in hierarchical S-expression format. This is a more natural syntax
for a ".s" file format from the perspective of LLVM MC and related tools,
while remaining easy to translate into other forms as needed.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@249364 91177308-0d34-0410-b5e6-96231b3b80d8