[fuzzer] Add basic support for emscripten.

Summary:
Add basic support for emscripten.

This enables libFuzzer to build (using build.sh) for emscripten and fuzz
a target compiled with
-fsanitize-coverage=inline-8bit-counters.

Basic fuzzing and bug finding work with this commit.
RSS limit and timeouts will not work because they depend on system
functions that are not implemented/widely supported in emscripten.

Reviewers: kcc, vitalybuka, hctim

Reviewed By: hctim

Subscribers: #sanitizers, llvm-commits

Tags: #sanitizers, #llvm

Differential Revision: https://reviews.llvm.org/D71285