All tests should now have annotation for *why* they're ignored on emscripten. A
few tests no longer need such an annotation as well!

Closes #41299

Resolves #43235.

Added test cases to cover all special-cased branches of udivmodti4.

Even more SNAP cleanup.

This commit adds a new entry to the Travis matrix which will execute emscripten
test suites. Along the way it updates a few bits of the test suite to continue
passing on emscripten, such as:

* Ignoring i128/u128 tests as they're presumably just not working (didn't
  investigate as to why)
* Disabling a few process tests (not working on emscripten)
* Ignore some num tests in libstd (#39119)
* Fix some warnings when compiling

just as ceil(log10(2^64)) == 20

This commit includes manual merge conflict resolution changes from a rebase by @est31.

This commit includes manual merge conflict resolution changes from a rebase by @est31.

Dangling a carrot in front of a donkey.

This commit includes manual merge conflict resolution changes from a rebase by @est31.