[WebAssembly] Enforce assembler emits to streamer in order.

[WebAssembly] Enforce assembler emits to streamer in order.

Summary:
The assembler processes directives and instructions in whatever order
they are in the file, then directly emits them to the streamer. This
could cause badly written (or generated) .s files to produce
incorrect binaries.

It now has state that tracks what it has most recently seen, to
enforce they are emitted in a given order that always produces
correct wasm binaries.

Also added a new test that compares obj2yaml output from llc (the
backend) to that going via .s and the assembler to ensure both paths
generate the same binaries.

The features this test covers could be extended.

Passes all wasm Lit tests.

Fixes: https://bugs.llvm.org/show_bug.cgi?id=39557

Reviewers: sbc100, dschuff, aheejin

Subscribers: jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D55149

llvm-svn: 348185