[WebAssembly] Implement build-id feature

Implement the --build-id flag similarly to ELF, and generate a build_id
section according to the WebAssembly tool convention specified in
https://github.com/WebAssembly/tool-conventions/pull/183

The default style ("fast" aka "tree") hashes the contents of the output
and (unlike ELF) generates a v5 UUID based on the hash (using a random
namespace).
It also supports generating a random v4 UUID, a sha1 hash,
and a user-specified string (as ELF does).

Differential Revision: https://reviews.llvm.org/D107662