[WebAssembly] Forbid use of EM_ASM with setjmp/longjmp

Summary:
We tried to support EM_ASM with setjmp/longjmp in binaryen. But with dynamic
linking thrown into the mix, the code is no longer understandable and cannot
be maintained. We also discovered more bugs in the EM_ASM handling code.

To ensure maintainability and correctness of the binaryen code, EM_ASM will
no longer be supported with setjmp/longjmp. This is probably fine since the
support was added recently and haven't be published.

Reviewers: tlively, sbc100, jgravelle-google, kripken

Reviewed By: tlively, kripken

Subscribers: dschuff, hiraditya, aheejin, sunfish, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D66356

llvm-svn: 369137