[WebAssembly] Add the signature for the new llround builtin function

r360889 added new llround builtin functions. This patch adds their
signatures for the WebAssembly backend.

It also adds wasm32 support to utils/update_llc_test_checks.py, since
that's the script other targets are using for their testcases for this
feature.

Differential Revision: https://reviews.llvm.org/D62207

llvm-svn: 361327