[WebAssembly] Fix p2align in assembler.
- Match the syntax output by InstPrinter.
- Fix it always emitting 0 for align. Had to work around fact that opcode is not available for GetDefaultP2Align while parsing.
- Updated tests that were erroneously happy with a p2align=0
Reviewers: aheejin, sbc100
Subscribers: jgravelle-google, sunfish, jfb, llvm-commits, dschuff
Differential Revision: https://reviews.llvm.org/D63633